Tomorrow is Easter, a holiday that is widely celebrated by Christians around the world. The event is linked to the crucifixion and resurrection of Christ, both of which occurred nearly two thousand years ago. While the Sunday celebration of Easter is central to a Christian belief, some may wonder why the religious holiday changes from year to year.
Passover and Easter
Easter varies from year to year because the early Christian fathers wished to correlate the holiday with the Jewish celebration of Passover. In doing this it was necessary to take in account the lunar cycle, which plays an important role in determining the occurrence of Jewish historical and holiday dates. Because of this, the actual date of Easter can vary from March 22 through April 25.
A More Detailed Explanation
In general, Easter falls approximately on the first Sunday after the first full moon following the equinox. I use the word approximate because there is something called the Paschal Full moon, which is not quite the same as the vernal equinox full moon. In fact the Paschal Full Moon uses the date of the vernal equinox as it occurred in the year 328 A.D., which just happened to be March 20. Therefore all Easter Sundays are based on the arbitrary rule that the equinox happens on March 20, though in reality the equinox may occur as late as March 22. To further complicate things Orthodox Christians use a different calendar for calculating Easter. In their world Easter may fall between April 3 and the first week in May. The Orthodox Easter also varies from Passover and the western Christian date of Easter.