This is the most important holy day of the Christian calendar. Easter Sunday commemorates the day in the springtime when the resurrection of Jesus is believed to have occurred. The date is calculated by one formula by most Eastern Orthodox churches, and by another formula elsewhere in Christianity. Easter Sunday falls on the first Sunday after the first full moon after MAR-20, the nominal date of the Spring Equinox. It can be on any Sunday from March 22 to April 25th. Eastern Orthodox churches sometimes celebrate Easter on the same day as the rest of Christianity. However if that date does not follow Passover, then the Orthodox churches delay their Easter - sometimes by over a month.