feastdate

Old parish registers often date an entry by the church year rather than the calendar: baptised Dom. Palm. 1656, buried Dom. 9. Trin., married Fer. 2. Pent. feastdate turns those into calendar dates, gives the weekday, and says which calendar the date is in.

$ feastdate "Dom. Palm. 1656"
Dom. Palm. 1656 = Sun 30 Mar 1656 (Julian)   [palm]

$ feastdate "Dom. 9. Trin." 1657
Dom. 9. Trin. 1657 = Sun 26 Jul 1657 (Julian)   [9. Sunday after Trinity]

$ feastdate "Fer. 2. Pent. 1724"
Fer. 2. Pent. 1724 = Mon 29 May 1724 (Gregorian)   [pent + 1 day(s)]

It is pure Python, uses only the standard library, and runs on Python 3.9 or later.

Install

pip install feastdate

Or the latest main straight from GitHub:

pip install git+https://github.com/isaacschepp/feastdate

This gives you a feastdate command, and python -m feastdate does the same thing.

Command line

$ feastdate "Dom XXIII post Trin. 1690"
Dom XXIII post Trin. 1690 = Sun 23 Nov 1690 (Julian)   [23. Sunday after Trinity]

$ feastdate "Die Viridium 1724"
Die Viridium 1724 = Thu 6 Apr 1724 (Gregorian)   [viridium]

$ feastdate "Festo Michaelis 1699"
Festo Michaelis 1699 = Fri 29 Sep 1699 (Julian)   [fixed feast 29 Sep]

$ feastdate --easter 1744
Easter 1744 = Sun 29 Mar 1744 (Gregorian)

$ feastdate --gregorian --easter 1744
Easter 1744 = Sun 5 Apr 1744 (Gregorian)

$ feastdate --gregorian "Dom. 1. Adv. 1650"
Dom. 1. Adv. 1650 = Sun 27 Nov 1650 (Gregorian)   [1. Sunday of Advent]

$ feastdate --switch england "Michaelis 1752"
Michaelis 1752 = Fri 29 Sep 1752 (Gregorian)   [fixed feast 29 Sep]

The reverse: what did the register call this day?

An index gives 26 Jul 1657, and you want to know whether the register's Dom. 9. Trin. agrees. --date names the day:

$ feastdate --date 1657-07-26
26 Jul 1657 (Julian) = Sun   Dom. 9. p. Trin. (9th Sunday after Trinity)

$ feastdate --date 1724-04-06
6 Apr 1724 (Gregorian) = Thu   Die Viridium (Maundy Thursday)

$ feastdate --date 1680-04-14
14 Apr 1680 (Julian) = Wed   Feria 4 post Pascha (Wednesday after Easter)

$ feastdate --date 1656-11-30
30 Nov 1656 (Julian) = Sun   Dom. 1. Adv. (1st Sunday of Advent); Andreae (St Andrew)

$ feastdate --date 1657-07-28
28 Jul 1657 (Julian) = Tue   no name of its own: the week of Dom. 9. p. Trin.

The date is read in the calendar in force on it, the same rule the forward direction uses: under the default, Julian before 1 Mar 1700, so --date 1700-02-24 is refused as a day that never happened. --gregorian, --julian and --switch apply to the date as well.

Every name printed is in a form feastdate reads back to the same day, followed by an English gloss. A day can have several names. Every Sunday has at least one, and so does every day from Septuagesima to the week of Trinity (a weekday there is a feria counted from its Sunday: Feria 4 post Jubilate). A weekday in the rest of the year has a name only if it is a saint's day; otherwise the output says which week it is in. That line is a description, not a register form: a feria is counted only from a Sunday without a number, so Feria 3 post Dom. 9. p. Trin. is not something feastdate reads. Corpus Christi and the Sundays after Pentecost are named under --gregorian and --switch catholic only, and Totensonntag under the default (and --switch prussia-duchy) from 1816.

You can put the year in the text or give it as a separate argument. A year inside the text is recognised from 1500 to 1899, so that a number such as the 9 in Dom. 9. Trin. is read as an ordinal. Give any other year, from 100 to 9999, as a separate last argument: a shorter number there is still read as an ordinal, since feastdate "Dom. Trin." 9 could mean either. (--easter takes any year from 1 to 9999, and so does the year argument of the Python API.) The bracketed part of the output shows what the parser recognised, so you can check it read the entry the way you meant. If it recognises nothing, it prints an error and exits with status 2.

Option	Meaning
(none)	Protestant German calendar (see below)
--gregorian	Gregorian throughout, for Catholic parishes
--julian	Julian throughout
--switch TERRITORY	another territory's switch: england, sweden, ... or LAST:FIRST (see below)
--easter YEAR	Easter Sunday of that year
--date YYYY-MM-DD	the reverse: the church-year names of that day
--iso	only the date and calendar letter: 1656-03-30 J
--json	one JSON object per entry
-	read one entry per line from stdin

Many entries at once, and output for scripts

--iso prints only the date, as YYYY-MM-DD followed by J (Julian) or G (Gregorian). --json prints one object per entry. A refusal is an object with an error key instead, and the exit status is still 2:

$ feastdate --iso "Dom. Palm. 1656"
1656-03-30 J

$ feastdate --json "Dom. Palm. 1656"
{"input": "Dom. Palm. 1656", "date": "1656-03-30", "calendar": "Julian", "weekday": "Sun", "jdn": 2326001, "parsed": "palm"}

$ feastdate --json "Dom. Palm."
{"input": "Dom. Palm.", "error": "no year: give one, e.g. \"Dom. Palm. 1656\""}

jdn is the Julian Day Number, the same day whichever calendar printed it. --json works with --easter and with --date, which gives the names as a list of name and gloss pairs.

To convert a whole extraction, give - as the text, or no text with stdin redirected, and put one entry on each line. If the year is kept in its own column, put it after a tab. Every input line gives one output line, including a refused line, so the output lines up with the input. Each refusal is also reported on stderr with its line number. The exit status is 0 only if every line resolved, and 2 otherwise:

$ printf 'Dom. Palm. 1656\nDom. Palm.\nDom. 9. Trin.\t1950\n' | feastdate --iso -
1656-03-30 J
feastdate: line 2: no year: give one, e.g. "Dom. Palm. 1656"
error: no year: give one, e.g. "Dom. Palm. 1656"
1950-08-06 G

The default single-entry output line has not changed.

Python

>>> from feastdate import feast_date, resolve, easter, fmt, name_of
>>> feast_date("Dom. Lætare", 1680)
'Sun 21 Mar 1680 (Julian)'
>>> feast_date("Pasch.", 1744, cal="G")
'Sun 5 Apr 1744 (Gregorian)'
>>> resolve("Whit Monday 1735")          # (Julian Day Number, what was recognised)
(2354905, 'whit + 1 day(s)')
>>> fmt(easter(1656))
'6 Apr 1656'
>>> name_of(1656, 11, 30)                # the reverse lookup
[('Dom. 1. Adv.', '1st Sunday of Advent'), ('Andreae', 'St Andrew')]

Function	Returns
feast_date(text, year=None, cal='P')	the formatted date, e.g. 'Sun 30 Mar 1656 (Julian)'
resolve(text, year=None, cal='P')	(julian_day_number, description); raises FeastError
name_of(y, m, d, cal='P')	every name of that day, [(name, gloss), ...], movable days first; each one resolves back to the day
week_of(y, m, d, cal='P')	(weekday, name of the Sunday that begins its week), e.g. ('Tue', 'Dom. 9. p. Trin.')
ymd(jdn, cal)	a Julian Day Number as (y, m, d) in the calendar in force on that day
show(jdn, cal)	a Julian Day Number formatted in the calendar in force on that day
easter(y)	Easter Sunday as a Julian Day Number, by the Protestant German reckoning
easter_of(y, cal)	Easter Sunday under cal ('P', 'G', 'J' or a switch)
calendar_of(cal)	cal as the module takes it: 'G', 'J' or a Switch; raises FeastError for an unknown name or a pair of days that are not consecutive
SWITCHES, Switch	the named switches, and the class to build your own
is_greg(jdn, cal)	whether that day is reckoned in the Gregorian calendar under cal
julian_easter(y), greg_easter(y)	(month, day) of Easter in that calendar
j2o, o2j, g2o, o2g	Julian or Gregorian (y, m, d) to and from a Julian Day Number
to_ord(y, m, d, cal)	a fixed date as a Julian Day Number, in the calendar in force on that day; raises FeastError for a day that never existed (19 to 29 Feb 1700 under 'P')
cal_of_year(y, cal, m=None, d=None)	'J' or 'G': the calendar a fixed date is reckoned in
is_leap(y, c)	whether y is a leap year in calendar c ('J' or 'G')
fmt(jdn)	'30 Mar 1656': Julian before 1 Mar 1700, Gregorian from it
FE	movable feasts as day offsets from Easter Sunday, e.g. FE['Trinity'] == 56

Every function that takes cal also takes a switch name (cal='england'), an explicit '1752-09-02:1752-09-14', or a Switch.

All dates are carried internally as Julian Day Numbers, so both calendars share one number line, and jdn % 7 gives the weekday (0 is Monday and 6 is Sunday).

The calendar rules

The default ('P') follows the Protestant German estates, such as Hessen-Darmstadt, Baden-Durlach and Brandenburg-Prussia:

• Julian to the end of 1699.

• The Improved Calendar from 1700. 18 February 1700 (Julian) was followed by 1 March 1700, which dropped eleven days. From then on the calendar dates are Gregorian.

• Easter in 1724 and 1744 is the exception. The Improved Calendar computed Easter astronomically, not from the Gregorian tables. Its result matched the Gregorian Easter in every year except these two, when the Protestants kept Easter a week earlier:

  Year	Protestant Easter	Gregorian Easter
  1724	9 April	16 April
  1744	29 March	5 April

  Every movable feast in those two years moves with it. Parish registers confirm this: a Hessian register dates Maundy Thursday 1724 (Die Viridium) to 6 April.

--gregorian ('G') is for Catholic parishes. Catholic territories adopted the Gregorian calendar in 1582 or 1583. Before that year the option still computes Gregorian dates, so for an earlier Catholic entry use the default or --julian.

--julian ('J') uses the Julian calendar for every year.

Every result states its calendar, (Julian) or (Gregorian), because a date without one is ambiguous for anything before 1700.

Other territories: --switch

Other territories changed calendar at other times. --switch takes one of these, or the two days of any switch written LAST:FIRST: the last Julian day, then the first Gregorian day.

--switch	Last Julian day	First Gregorian day	Easter
de-protestant	18 Feb 1700	1 Mar 1700	the default, 'P': 9 Apr 1724 and 29 Mar 1744
denmark-norway	18 Feb 1700	1 Mar 1700	as de-protestant, 1724 and 1744 included
prussia-duchy	22 Aug 1610	2 Sep 1610	Gregorian from 1611; 1724 and 1744 are refused
sweden	17 Feb 1753	1 Mar 1753	astronomical from 1740: 18 Mar 1744 (Julian), 25 Apr 1802, 21 Apr 1805, 29 Mar 1818; 1 Mar 1700 to 30 Feb 1712 is refused
england, british-colonies	2 Sep 1752	14 Sep 1752	Gregorian from 1753
catholic	4 Oct 1582	15 Oct 1582	Gregorian from 1583

$ feastdate --switch 1752-09-02:1752-09-14 "Dom. 15. p. Trin." 1752
Dom. 15. p. Trin. 1752 = Sun 17 Sep 1752 (Gregorian)   [15. Sunday after Trinity]

The rules for every switch:

• A date is read in the calendar in force on that day, as under the default: Michaelis 1752 under england is Gregorian 29 September, and Pasch. 1752 is Julian 29 March. The days a switch dropped (3 to 13 September 1752 under england) are refused.
• Easter is Julian before the switch and Gregorian after it. In the year of the switch the Gregorian Easter is used only if it falls on or after the first Gregorian day. So England kept a Julian Easter in 1752, and Sweden a Gregorian one in 1753.
• The two days of LAST:FIRST must be consecutive, or the switch is refused: the day after Julian 2 Sep 1752 is Gregorian 14 Sep, so 1752-09-02:1752-09-13 is an error. An explicit switch has no Easter exceptions.

What each preset models, and why:

• denmark-norway changed with the Protestant estates of the Empire, and kept the same astronomical Easter of the Improved Calendar, so its Easter was a week before the Gregorian one in 1724 and 1744 too.
• prussia-duchy (Ducal, later East, Prussia) followed its Polish overlord in 1610. It is not established here whether it then kept the Protestant Easter of 1724 and 1744 with the other Protestant estates, or the Gregorian Easter it had kept since 1611. Those two Easters, and every movable feast of those two years, are refused. Pick --switch de-protestant or --gregorian once you know which the parish kept.
• sweden (with Finland until 1809) is the most complicated. From 1 Mar 1700 to 30 Feb 1712 Sweden used its own calendar, one day ahead of the Julian. feastdate does not model that calendar, and refuses every date in it, rather than give a weekday that is a day off. From 1740, still in the Julian calendar, Sweden computed Easter astronomically as the Improved Calendar did. It kept that until 1844, so its Easter was a week before the Gregorian one in 1744 (18 Mar 1744, Julian) and a week after it in 1802, 1805 and 1818. By the same computation 1825 and 1829 should have been late too, but Sweden kept the Gregorian date in those years, and so does feastdate. Finland after 1809 is not modelled.
• england follows the Calendar (New Style) Act 1750, for Great Britain and its colonies. The same Act moved the start of the year from 25 March to 1 January, from 1752. Before then an English date from 1 January to 24 March is written with two years, 1680/81. feastdate reads a year the modern way, starting 1 January, so give the later year: Candlemas 1680/81 is Purif. 1681.
• catholic is the switch of Inter gravissimas, followed by Spain, Portugal, Poland and most of Italy. Many Catholic territories changed a few weeks or years later (France in December 1582, most Catholic German states in 1583 to 1585): give those as LAST:FIRST. Unlike --gregorian, it is Julian before the switch.
• The Protestant Swiss cantons changed in 1701, 1724 and as late as 1812, canton by canton. There is no preset for them: give the canton's two days as LAST:FIRST.

Sources: the switch days are those of the list of Gregorian adoption dates and Claus Tøndering's calendar tables. The Easter dates of the Improved Calendar, and the territories that kept them in 1724, 1744, 1802, 1805, 1818, 1825 and 1829, are from R. H. van Gent's table of anomalous Easter dates (Utrecht University), and the Swedish ones also from the Swedish calendar article. These are secondary sources. Grotefend's Zeitrechnung is the standard reference, and a date it gives differently is a bug worth filing.

Accepted spellings

The parser tolerates the abbreviations and punctuation registers actually use. Case is ignored, æ/ä/ö/ü/ß are folded, and punctuation is ignored. Each word matches by its beginning, so Reminisc., Reminisc: and Reminiscere all mean the same Sunday.

Day	Offset from Easter	Recognised by the word beginning with
Septuagesima	−63	septuag
Sexagesima	−56	sexag
Estomihi / Quinquagesima	−49	estomihi, esto, quinq, fastnacht
Shrove Tuesday	−47	fastnachtsdienstag, fastnacht dienstag (Estomihi and a weekday, below)
Ash Wednesday	−46	aschermittw, cinerum (Dies Cinerum, Feria IV Cinerum), ash wednes
Invocavit	−42	invoc
Reminiscere	−35	reminisc
Oculi	−28	oculi
Laetare	−21	laetare, laet, lat (so Lætare, Lätare)
Judica	−14	judica
Palm Sunday	−7	palm
Maundy Thursday	−3	viridium, coena, cena, grundonnerstag, maundy
Good Friday	−2	parasceve, karfreitag, charfreitag, good
Holy Saturday	−1	karsamstag, charsamstag, karsonnabend, ostersamstag, ostersonnabend, oster samstag, sabbat sanct (Sabbatho Sancto), holy sat
Easter	0	pasch, ostern, oster, easter
Quasimodogeniti	+7	quasimod, quasi, quas, weisser
Misericordias Domini	+14	miseric, miser
Jubilate	+21	jubil
Cantate	+28	cantat
Rogate / Vocem jucunditatis	+35	rogat, voc
Ascension	+39	ascens, himmelfahrt
Exaudi	+42	exaudi
Pentecost / Whitsun	+49	pentecost, pent, pfingst, whit
Trinity Sunday	+56	trinit, trin, dreifaltig
Corpus Christi	+60	fronleichnam, corp christi (Corporis Christi, Corpus Christi)

A name of two words, such as sabbat sanct, matches the two words in order, with only connecting words between them (see Fixed feasts below).

Ostersamstag and Ostersonnabend are the Saturday before Easter, not after it.

Corpus Christi is a Catholic feast. It resolves under every calendar, the default included, because the date is the same. A Protestant register naming it is usually dating by a neighbouring Catholic parish's calendar, so consider --gregorian.

Buß- und Bettag is not recognised, on purpose. Its date varied by territory and century, so no single rule gives it, and feastdate refuses it rather than guess.

Numbered Sundays take an Arabic or Roman ordinal, with or without an ending (9., 9th, IX, 2da):

• Dom. 9. Trin., Dom XXIII post Trin., 9th Sunday after Trinity give the nth Sunday after Trinity.

• Dom. 1. Adv. to Dom. 4. Adv. give the Sundays of Advent, counted back from Christmas.

• Dom. 5. p. Epiph. gives the nth Sunday after Epiphany. Epiphany alone gives 6 January.

• Dom. 5. post Pent., Dominica 5. post Pentecosten and Dom. 5. nach Pfingsten give the nth Sunday after Pentecost, the count of Catholic registers and of Protestant ones before the Trinity count took hold. Dom. 1. post Pent. is Trinity Sunday, so Dom. 5. post Pent. is Dom. 4. p. Trin.. A year has 23 to 28 of them, one more than the Sundays after Trinity.

• Dom. ult. p. Trin., Dom. ult. Trin., letzter Sonntag nach Trinitatis and Dom. ultima post Pent. give the last Sunday after Trinity or Pentecost, a week before Advent 1: in 1680, Dom. 24. p. Trin..

• Totensonntag and Ewigkeitssonntag are the same Sunday, but Prussia named it only in 1816, so Totensonntag 1815 is refused. Before 1816, write Dom. ult. p. Trin..

• The count is checked against the year. The Sundays after Trinity run up to Advent, so a year has 22 to 27 of them (23 to 28 counted after Pentecost), and the Sundays after Epiphany run up to Septuagesima, 1 to 6. A Sunday the year did not have is refused, not carried into Advent or Lent:

  $ feastdate "Dom. 26. Trin. 1680"
  feastdate: there were 24 Sundays after Trinity in 1680, not 26 in 'Dom. 26. Trin. 1680'

Weekdays after a feast:

• Fer. 2. Pent., Feria 2da Paschat. and Fer. 3. Pasch. use the feria count, where 2 is Monday and 3 is Tuesday. A feria outside 1 to 7 is refused.
• Whit Monday, Easter Tuesday, Ostermontag and Pfingstdienstag are also understood, with the weekday written as part of the word or as a separate word. The weekday counts from a Sunday feast only: Fastnachtsdienstag is Estomihi plus two days, and Karfreitag Montag is refused.
• Feria secunda, Fer. tertia spell the number out; prima to septima are understood.
• With a feast that is not a Sunday, the feria names that feast's own weekday, where 1 is Sunday: Feria 6 in Parasceve is Good Friday and Feria V in Coena Domini is Maundy Thursday. Nothing is added to the date. The number is checked instead, so Fer. 2. Ascens. is refused, because Ascension is a Thursday.

A day counted from a feast: the day stands before nach, post, p. or after (or vor, ante, before), and the feast after it. The answer is the first such day strictly after the feast, or the last one strictly before it:

$ feastdate "Freitag nach Jubilate 1680"
Freitag nach Jubilate 1680 = Fri 7 May 1680 (Julian)   [Friday after jubilate]

• Freitag nach Jubilate, Mittwoch nach Oculi, Montag nach Trinitatis and Sonnabend vor Palmarum name the weekday. Feria 4 post Oculi and Fer. 6 p. Reminisc. name it by the feria, where 1 is Sunday.
• Dom. names the Sunday: Dom. p. Nativ., Sonntag nach Michaelis, Dom. post Martini, Dom. ante Circumcis.. Strictly after means that when Michaelmas is itself a Sunday, as in 1689, Sonntag nach Michaelis is the Sunday a week later.
• Dom. post Pascha and Dom. 1. post Pascha are Quasimodogeniti, and Dom. 2. p. Pasch. is Misericordias, up to Dom. 6. post Pascha. Easter, Pentecost, Trinity and Epiphany are the only feasts whose Sundays are numbered (see Numbered Sundays above).
• The year is the feast's: Dom. post Nativ. 1740 is Sunday 1 January 1741.
• A weekday word after the feast still checks or moves the feast, so Freitag nach Ostermontag is the Friday after Easter Monday. One nach at a time: Freitag nach Dom. post Oculi is refused.

Days of a German feast: Easter, Whitsun and Christmas were kept over several days, and German registers count them:

• 2. Ostertag, der 2te Ostertag, 2ter or zweiter Ostertag is Easter Monday, and 3. Ostertag or der dritte Ostertag is Easter Tuesday. Osterfeiertag and Oster Tag are the same.
• 2. Pfingsttag is Whit Monday. 2. Weihnachtstag and 2. Christtag are 26 December.
• letzter Ostertag is the third day, as the registers use it.
• A feast was kept three days at most, so 4. Ostertag is refused.

Fixed feasts give you the weekday too. A name of two words, such as mariae verk, matches those two words in order, with only connecting words (et, und, S.) between them, and it wins over a one-word name on either word: Nativ. Mariae is 8 September, not Christmas, Mariae Himmelfahrt is 15 August, not Ascension, and Johannis Evang. is 27 December, not 24 June.

Feast	Date	Recognised by
Circumcision / New Year	1 Jan	circumcis, neujahr
Conversion of St Paul	25 Jan	pauli bekehr, bekehr pauli, convers pauli
Purification (Candlemas)	2 Feb	purif, lichtmess
St Matthias	24 Feb, 25 Feb in a leap year	matthia, mathia
St Gregory	12 Mar	gregor
Annunciation	25 Mar	annunc, mariae verk
St George	23 Apr	georg
SS Philip and James / Walpurgis	1 May	philippi jac, phil jac, walpurg
Finding of the Cross	3 May	kreuzerfind, kreuz erfind, invent cruc
St John the Baptist	24 Jun	johannis, joh bapt
SS Peter and Paul	29 Jun	petri pauli, peter paul
Visitation	2 Jul	visitat, heimsuch
St Mary Magdalene	22 Jul	magdalen
St James	25 Jul	jacob
St Lawrence	10 Aug	laurent
Assumption	15 Aug	assumpt, mariae himmelf, himmelf mariae
St Bartholomew	24 Aug	bartholom
Nativity of Mary	8 Sep	nativ mariae, mariae geburt
Exaltation of the Cross	14 Sep	kreuzerhoh, kreuz erhoh, exalt cruc
St Matthew	21 Sep	matthae, matthai, matthau, mathae
Michaelmas	29 Sep	michael
St Gall	16 Oct	galli, gallus
SS Simon and Jude	28 Oct	simon jud
All Saints	1 Nov	omnium sanct, allerheilig
All Souls	2 Nov	omnium anim, allerseel
Martinmas	11 Nov	martini
St Elisabeth	19 Nov	elisab
St Andrew	30 Nov	andreae, andreas
St Thomas	21 Dec	thomae, thomas
Christmas	25 Dec	nativ, christtag, weihnacht, christmas
St Stephen	26 Dec	stephan
St John the Evangelist	27 Dec	johannis evang, joh evang
Holy Innocents	28 Dec	innocent, unschuld, kindlein

A saint's epithet is a connecting word: Andreae Apost., Matthaei Apost. et Evang., Michaelis Archangeli, Laurentii Martyr. (apost, evang, archang, martyr, virg, episc).

St Matthias and the leap day. The Julian calendar counted its leap day in by doubling 24 February (the bissextile day), so in a leap year the feasts after it moved a day later, and German almanacs, Protestant and Catholic, kept Matthias on 25 February in a leap year long after 1700. feastdate follows that reckoning for any feast from 24 to 28 February, which in this table is Matthias alone. The leap year is the one of the calendar in force on the day: 1800 is a leap year in the Julian calendar but not in the Gregorian, so Matthiae 1800 is 24 February by default and 25 February with --julian. The label says when the rule applied:

$ feastdate "Matthiae 1680"
Matthiae 1680 = Wed 25 Feb 1680 (Julian)   [fixed feast 25 Feb, leap year]

Under the default rules there was no St Matthias in 1700, since 18 February (Julian) was followed by 1 March, and Matthiae 1700 is refused.

Nothing is skipped. Every word and number in the text must be accounted for: a feast name, a number the feast uses, a weekday that agrees with the answer, a day counted from the feast (Freitag nach), or a connecting word (Dom., Festo, der, Domini and the like). Anything else is an error, never dropped, so the answer is not a plausible date that ignored part of what you typed:

$ feastdate "2. Ostern 1747"
feastdate: the number 2 is not used by ostern in '2. Ostern 1747': a day of the feast is written "2. Ostertag" or "Fer. 2. Pasch."

$ feastdate "Dom. Oculi = 4 March 1638"
feastdate: unrecognised word in 'Dom. Oculi = 4 March 1638': march

Pass the feast name alone, not the whole sentence around it. A weekday word is a check, unless it stands before nach / post / vor: Good Friday must fall on a Friday, and Dominica Viridium is refused, because Maundy Thursday is not a Sunday.

A year written in the text must fall between 1500 and 1899, so that other numbers are read as ordinals. The year argument accepts any year.

Why this is tricky

• There are two calendars, and the switch between them depends on the territory. The same Sunday is ten or eleven days apart depending on which calendar the parish kept, and neighbouring Catholic and Protestant parishes differed for more than a century.
• Easter is not a fixed date, and there are three Easter rules: the Julian computus, the Gregorian tables, and the Protestant astronomical reckoning. The last differs from the Gregorian Easter only in 1724 and 1744, so an off-the-shelf Gregorian Easter function is silently wrong for a German Protestant register in exactly those two years.
• The year 1700 is split. For a Protestant parish, January and most of February 1700 are Julian and the rest of the year is Gregorian, so a date in 1700 has to know which side of 18 February it falls on.
• Advent and Epiphany Sundays hang off fixed dates, not Easter. Advent 1 is the fourth Sunday before Christmas Day, so it falls between 27 November and 3 December. A Sunday "after Epiphany" is strictly after 6 January, so when 6 January is itself a Sunday, the first Sunday after Epiphany is the 13th.
• Registers abbreviate freely (Dom. Miser. Dni., Dom. Voc. jucund.), mix Latin and German, and write ordinals in Roman or Arabic numerals with Latin endings.
• A hand-copied Easter routine drifts without anything to show it. The test suite checks dates against days the registers themselves settle, and checks that every computed Sunday in 1600–1799, in all three calendars, falls on a Sunday. It also runs the reverse lookup over every day of those two centuries and resolves each name it gives back to the same day, so the two directions cannot drift apart.

Development

pip install -e ".[test]"
pytest

License

MIT. See LICENSE.