Easter this year lands on March 27, a relatively early date but perfect timing for anyone who needs a sugar fix since crashing shortly after Valentine's Day. Next year, April 16 marks the holiday.
Easter moves around year-to-year and can occur anywhere between March 22 and April 25. But why does it always fall on a different day within that five-week period? Surely, it's not simply to keep Easter-egg hunters on their toes?
The answer boils down to the difference between solar and lunar cycles. The ancient Egyptians first developed the solar calendar, which the Romans adopted, creating the Julian calendar. The Gregorian calendar succeeded the Julian calendar in Western Europe, and is the global standard today. The Hebrew calendar, however, follows the lunar cycles, as does the Islamic calendar.
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Easter is a religious holiday that commemorates the resurrection of Jesus Christ and always occurs on a Sunday. The preceding Friday marks Good Friday, the day of Jesus' death. And the night before that was the Last Supper, a feast for the Jewish holiday of Passover, according to the Bible. Given that it's a Jewish holiday, Passover naturally follows the Hebrew calendar.
The solar and lunar calendars don't sync up. The solar calendar is 365 days, five hours and 49 minutes, which is why February has a leap day every four years, while the lunar calendar only has 354 days. Reconciling the differences between the two has been attempted several times over the centuries, and different religious traditions have adopted their own timelines for observing the holiday.
In 325 A.D., the Council of Nicea convened and, in addition to hammering out the basic principles of their still flowering faith, attempted to set a standard for the celebration of the religious holiday. The council, composed of Christian bishops, decided that the church would observe Easter on the Sunday after the first full moon on or after the vernal equinox, known as the Paschal full moon. If the first full moon occurs on a Sunday, then the holiday is pushed back a week so that it still takes place after Passover.
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Although not universally observed at the time or the centuries that followed, the council established the basic practice for how Easter would be observed. They did make a number of errors in their calculations, however. The bishops fixed the date of the vernal equinox at March 21, though it occurs on March 20 today. Also, instead of relying on astronomical observations, the council instead devised a set of tables intended to following the lunar cycle that doesn't sync up with the phases of the moon.
That explains why the date of Easter moves every year. But the day most Western Christian churches celebrate Easter doesn't match the date of the holiday as observed by the Eastern Orthodox church. Again the discrepancy comes down to calendars. The Orthodox church still follow the Julian calendar, which is around 11 days behind the Gregorian calendar. Orthodox tradition observes Easter anywhere between April 4 and May 8. This year, it falls on May 1.
Attempts to modify the timing or set a fixed date for Easter have been welcomed by a number of Christian denominations, including the Catholic Church. One of the most recent efforts occurred in 1997 when representatives of various denominations convened in Aleppo, but failed to adopt a new standard.