The practice of deciding when Easter falls is a tradition millennia old yet involves a pagan practice: that of the moon’s cycle through the heavens.
Constantin the Great, a Roman Emperor, decreed in 325, that Easter be celebrated on the first Sunday after the full moon following the vernal equinox. (The vernal equinox occurs twice a year when night and day are the same length.) By using the Julian calendar, each year was 11 minutes and 14 secs longer than the solar year. Over the years, this caused distinct discrepancies in Western Church celebrations of Easter.
Given the conflict between the solar and lunar years, and between the calendic and true astronomical years, the church adopted a system of calculations proposed by astronomer, Victorinius in 465.
British and Celtic Christian churches refused to comply, leading to disagreements with Rome. At this time, the Julian calendar was in use, but in 1582, Pope Gregory XIII reformed the calendar into the Gregorian calendar which dissolved most of the controversy by dropping ten days off the calendar. Western Christian churches were then be able to celebrate Easter at the same time; Eastern Christian Churches however, chose not to adopt the new system and so celebrate Easter either a week before or after the dates of the West.
Because of the difference between the Lunar and Solar cycles, Easter has become ‘a moveable feast’.
The word ‘Easter’ is thought to be pre-Christian in origin, from the Anglo-Saxon for the Teutonic goddess of spring and fertility, Eastre. According to St Bede – an 8th Century British scholar – the month of April was sacred to her. The vernal equinox is her day of celebration. Easter Eggs, too, are a part of the festival, representing fertility.
Historically speaking, Rome, in an effort to suppress Pagan rituals, changed iconic Christian celebratory days to those of Pagan worship; it made the conversion from paganism to Christendom easier for the Church. And yet, the church also integrated paganism into the Christian doctrine and calls it their own.
The remains of ancient Roman temples and sites of Celtic significance have been found under a number of Christian churches across Europe, including the Notre Dame in Paris.
It is a curious hypocrisy of the church that Christmas – the day the Church says Jesus Christ was born – is on a set date, immovable (though some astronomers believe the date is...erroneous due to the astronomical event at the time described in literature and the Bible which did not take place in December, but in April – other astronomical events were also recorded over a period of ten years, thus the true date is up for debate), but the day of his death changes due to the vagaries of the lunar cycle.