Easter was (and is) the central feast of the Christian calendar. The calculation of Easter was a complicated and disputed matter in Late Antiquity and the early Middle Ages. Disputes arose because it required calculation in advance, so that Lent could be observed. Broadly, all one needed to do was to identify the first Sunday after the first full moon after the Vernal Equinox. Yet lunar cycles (for the full moon) and solar cycles (for the Sunday) are not readily calculable without precise instruments or assorted mathematical ‘fixes’. Moreover, the two cycles do not closely relate to each other (the eventual best solution devised required a 532-year cycle of dates). A number of different models were devised, some better than others.
For Merovingian Gaul, the basic outline of what happened is as follows. By 541, most churches seem to have used the ‘Roman’ tables devised by mathematician Victorius of Aquitaine a century earlier. Indeed, at the Council of Orléans, Frankish bishops declared their preference for the table. The accuracy of the table was a concern, although the only known early critic – St Columbanus in c. 603/4 – was himself in the process of being condemned for a lack of canonical rigor when he made his initial objections known. Columbanus used a table popular in Ireland which allowed the Sunday and the Full Moon to fall on the same day – a situation condemned as judaizing since the first ecumenical council at Nicaea in 325 because of its association with Passover. Towards the end of the seventh century, a different table based on an Alexandrian (‘Greek’) one and translated by Dionysius Exiguus started to gain popularity (possibly earlier, but the evidence is far from clear enough to be certain). The methods of Victorius and Dionysius did not always produce the same Easter date but it seems that people were able to reach consensus until c. 740. By this point, the Frankish court was clearly using Dionysius’s work, supported by trends in Ireland and England. The Dionysiac Easter soon became the standard across the West.
The problem, as Immo Warntjes has pointed out, is that there is a significant volume of material on this process which has received little-to-no scholarly attention. There is a wealth of anonymous texts few people even know about. The narrative above is not definitive. In the meantime, here is a list of some of the more readily accessible ones:
Victorius of Aquitaine, Cursus paschalis, ed. T. Mommsen, MGH AA, 9 (Berlin, 1892), pp. 666-735 [link] – text completed in 457 at the request of Archdeacon Hilarius (later pope 461-8).
Dionysius Exiguus, Libellus de cycle magno paschae DCCCII annorum + Argumenta, ed. B. Krusch, in his Studien zur christlich-mittelalterlichen Chronologie 2 (Berlin, 1938), pp. 63-81 [link] – an attempt by a famous compiler and translator of canon law to solve the problem in 525 for the papacy. The text was later ‘updated’ by Cassiodorus c. 582. The edition as published by Krusch is confusing because it relegates some genuine parts of the text to footnotes and includes some sections which are obviously later additions in the main text. There is a partial translation and explanation here.
Computus Cottonianus of 688/9, part. ed. J. Gomez Pallares in ‘Hacia una nueva edicion de los Argumenta Paschalia de Dionisio el Exiguo’, Hispania Sacra, 46 (1994), 13-31. This text, known from the 743 manuscript London, Cotton Caligula, A xv [link], is an updated and expanded version of Dionysius’s text, possibly produced for the Frisian mission of St Willibrord (d. 739). Not coincidentally, one of the oldest extant Dionysian Easter tables survives from Willibrord’s circle as a page in Paris, Bibliotheque nationale, lat. 10837 [link], famous for including The Calendar of Willibrord.
Computus of 727, ed. A. Borst, MGH QQ zur Geistesgesch. 21. 1 (Hanover, 2006), pp. 348-74 [link] – a pro-Victorius text, known only from the contemporary section of the manuscript Bern, Burgerbibliothek, MS 611 [link]. It is written in a pre-Caroline minuscule with echoes of the Luxeuil school, which encouraged Borst to suggest the text was Burgundian.
Computus of 737, ed. A. Borst, MGH QQ zur Geistesgesch. 21. 1 (Hanover, 2006), pp. 375-423 [link] – the first full pro-Dionysian computus known to have been produced in Frankish circles (it refers to King Theuderich IV’s death in the ‘present year’). It has been associated with the circle of St Boniface and/ or Cologne. It is known only in the Veronese manuscript of c. 800 now Berlin, Staatsbibliothek, Phillipps 1831.