The emphasis on honoring tradition and maintaining an intact religious identity was therefore crucial."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." 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. 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.
This division between the Eastern and Western Churches has no strong theological basis, but neither is it simply a technical skirmish.As the World Council of Churches has noted, much of Orthodox Christianity is located in the Middle East, where it has frequently been the minority religion, and in Eastern Europe, where until recently it faced hostility from communist governments.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.
The Eastern Church sets the date of Easter according to the actual, astronomical full moon and the actual equinox as observed along the meridian of Jerusalem, site of the Crucifixion and Resurrection.
The Eastern Orthodox Church also applies the formula so that Easter always falls after Passover, since the Crucifixion and Resurrection of Christ took place after he entered Jerusalem to celebrate Passover.