Many people wonder why we often have different dates for Easter depending on which part of the Church we belong to. The answer comes from the way Easter is calculated. You may know that Easter Sunday falls on the first Sunday after the Full Moon on, or after, the March Equinox. The difficulty arises, however, because the date of the equinox actually varies from year to and from place to place.

So Western Christians in the fourth Century, at the Council of Nicaea agreed to define Easter with respect to a fixed equinox date of March 21. Thus Easter can easily be determined without worrying about fluctuations in the sun and moon from year to year. 

However the Eastern Orthodox churches decided not to follow that definition, preferring to determine Easter from the real equinox and full moon at the longitude of Jerusalem. Hence, the occasional difference in the date of Easter between churches. 

This is complicated further by the fact that the Eastern Churches still use the Julian calendar to calculate ecclesiastical dates. The Julian calendar is currently 13 days later than the Gregorian calendar. Since 1923, the Romanian Orthodox and Greek Orthodox churches have adopted the Gregorian calendar. However, they continue to use the Julian calendar for Easter calculations. 

If you're interested, here are the dates of Easter Sunday according to the Western calendar up to the year 202y.

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