One difficulty with Easter is that it is a “movable” feast: It occurs on different calendar dates, unlike a holiday like Christmas, which is always on December 25th. Indeed, Easter is the movable feast, since all the other ones (Ash Wednesday, Pentecost, etc.) move with it so as to keep the same relative position in the calendar. Easter moves as an artifact of originally having a position in the Jewish calendar. The Christian Last Supper, where the Orthodox and Catholics believe Jesus introduced the miracle of the Eucharist (a relatively trivial moment for most Protestants, though not for Martin Luther himself), was a Passover Seder. Passover begins on the 15th day of the month of Nîsân, which, like its namesake the Babylonian month of Nisannu, appears to move around relative to solar calendars because it follows the moon. The month begins at the time of the New Moon or shortly thereafter. Since 12 lunar months come to only about 354 days, an extra month must be added every two or three years to keep the calendar from running ahead of the sun and the seasons. This means that Jewish calendar dates move up steadily against solar dates and then abruptly jump back. This is what Passover does, and what Easter does also.
