A Tale of Two Easters

Easter is not only a movable holiday but a multiple one: in most years Western Christian churches and Eastern Orthodox churches celebrate Easter on different dates. In 2015, for example, Easter will be celebrated on April 5 by Western churches and April 12 by Orthodox churches. But in 2014, the two celebrations occurred on the same date, April 20.

The theological inconsistency of two Easters has remained a thorny problem for the Christian Church. "It has long been recognized that to celebrate this fundamental aspect of the Christian faith on different dates," states the World Council of Churches, "gives a divided witness and compromises the churches' credibility and effectiveness in bringing the Gospel to the world."

A Simple Formula, Complicated Interpretations

The formula for Easter "The first Sunday after the first full moon on or after the vernal equinox" is identical for both Western and Orthodox Easters, but the churches base the dates on different calendars: Western churches use the Gregorian calendar, the standard calendar for much of the world, and Orthodox churches use the older, Julian calendar.

That much is straightforward. But actually calculating these dates involves a bewildering array of ecclesiastical moons and paschal full moons, the astronomical equinox, and the fixed equinox and that's in addition to the two different calendar systems.

Source: https://www.infoplease.com/calendar-holidays/major-holidays/tale-two-easters