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. This year, however, is one of the rare occasions when the holiday falls on the same date for both ? April 11.

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 their dates on different calendars. Western churches use the Gregorian calendar ? a standard international calendar for civil use ? while the Orthodox churches use the older, Julian calendar.

That much is straightforward, but calculating these dates also involves a bewildering array of ecclesiastical moons and paschal full moons, the astronomical equinox, and the fixed equinox. The two churches vary on the definition of the vernal equinox and the full moon.

The Eastern church determines 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 the resurrection.

The Western church does not use the actual, or astronomically correct full moon but the ?ecclesiastical moon?, which is based on tables created by the church ? a system that allows the date of Easter to be calculated in advance.

The current Gregorian ecclesiastical rules that determine the date of Easter trace back to 325 CE at the First Council of Nicaea convened by the Roman Emperor Constantine, when the world used the Julian calendar put in place by Julius Caesar.

The Council decided to keep Easter on the same Sunday throughout the world, and to make it determinable indefinitely in advance through the use of special tables. In 1582 Pope Gregory XIII completed a reconstruction of the Julian calendar and produced new Easter tables. One major difference between the Gregorian and Julian calendars is the ?leap year rule?. By the 1700s, most of western Europe had adopted the Gregorian calendar.

For the Western Christian churches (Roman Catholic and Protestant), the ecclesiastical rules that determine the date of Easter mean that it can never occur before March 22 or later than April 25.

The theological inconsistency of two Easters has remained a thorny one for the Christian Church. A meeting of the Council of World Churches in 1997 proposed a solution thought to be favourable to East and West: both methods of calculating the equinox and the paschal full moon would be replaced with the most advanced, astronomically accurate calculations available, using the meridian of Jerusalem as the point of measure, but to date the issue remains unresolved.