Easter is the most important feast day of the Christian church. It is a moveable feast and the methods of calculation used to determine the date are very complex. Easter must always occur on a Sunday, as Sunday was the day of Christ’s resurrection. The date is calculated taking into account lunar and solar calendars. In the early church there were several methodologies used at different points in time to determine the date.
By the 5th and 6th centuries there was divergence within the church as to how to calculate Easter. In Ireland, northern and western Britain (an area which had strong links to the Irish church, as Irish missionaries had converted this area), the church used an eighty-four-year Easter cycle and assigned Easter to the Sunday that fell between the fourteenth and twentieth days of the lunar month. This system of calculation was at odds with the Continental church and the church in southern Britain (that had strong links with Rome). These churches used a system devised by Victorius of Aquitaine (457 AD) and from 630’s the Alexandrian system to calculate Easter.
With two systems for calculation in play divergent dates for Easter often occurred. This meant Easter could be celebrated at different times of the year in different geographical locations. In the mid 7th century Rome began moves to bring uniformity to the Christian church. This uniformity extended to a standard date for Easter across the Christian World.
In 630 Pope Honorius wrote to the Irish threatening excommunication if they did not conform to the Roman way of calculating Easter. As a result of the this letter the southern church in Ireland held a synod in 632 at Magh Lene or as it is known today Old Leighlin in Co Carlow. The synod probably took place at the site of Old Leighlin Cathedral which is built upon the ruins of an early monastic site founded by St Gobban in the early seventh century. At the time of the synod St Laserian who was the successor of Gobban was the leader of the religious community here.
Old Leighlin Cathedral (https://commons.wikimedia.org/wiki/File:Old_Leighlin_Cathedral_-_geograph.org.uk_-_417024.jpg)
The synod decided to send a delegation to Rome. Tradition holds that St Laserian was among the delegation. Upon their return the southern church decided to conform to the Roman Easter, however the northern church didn’t conform until it was accepted by the archbishop of Armagh in third quarter of the 7th century .
In Britain the famous synod of Whitby 664 settled the question in favor of Roman system but some clergy unhappy with the outcome came to Ireland. For example St Colmán the bishop of Lindisfarne along with a group of monks from Lindisfarne came to Ireland and set up a monastery on Inishbofin Co Galway and later at Mayo. It was not until 716 that the Columban federation of churches finally accepted Roman Easter and there was uniformity throughout the Christian world concerning Easter.
© Louise Nugent 2013
References
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Szarmach, P. 1996. Holy Men and Holy Women. Old English Prose Saints’ Lives and Their Contexts. Suny Press
http://www.saintsandstones.net/saints-inishbofin-journey.htm
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http://www.catholic.org/saints/saint.php?saint_id=4195
Pilgrimage In Medieval Ireland 
