What is the reason behind this? Easter is termed a moveable feast because it is not fixed in relation to the calendar. It falls on Sunday in late March 22 or April 25 and the next day is celebrated as Easter Monday. Historically the precise date of Easter has been a matter of contention.
After several centuries of disagreement, the First Council of Nicaea in 325 AD decided that all Christians would celebrate Easter on the same day - a Sunday. But there was no proof showing that the council reached an agreement on the exact date Easter should be celebrated. Following centuries of disagreement, all churches accepted the computation of the Alexandrian Church (now the Coptic Church) that Easter is the first Sunday after the first 14th day of the moon - the Paschal Full Moon that falls on or after March 21. But this still does not make it easy to determine the exact date of Easter, especially since it is mostly reliant on the full moon - the Paschal Full Moon in particular.
