Logic would say that Easter would fall on the first Sunday after the first day of Passover, or the Sunday during Passover week. After all, the gospels tell us that Jesus was crucified on or about Passover, and that he rose on the Sunday of Passover week. This year (2008) Easter fell on March 23rd. So why is Passover not until Sundown on April 19th? Why almost a month discrepancy between the two supposedly related events? (To complicate the issue, Eastern Orthodox Easter is April 27 in 2008.)
The discrepancy is related to the formula for calculating Easter and to the nature of the Jewish calendar. Easter, according to the Council of Nicea in 325 AD, should never fall on the first day of Passover. By this time, the Gentile world had gained priority over the Jewish membership of the church. In an apparent effort to assimilate Jewish Christians into the Gentile Church, they made sure that the two dates would never coincide. In a compromise between the Eastern (Byzantine/Syrian) and Western (Roman) factions, the date for Easter was set roughly as the first Sunday after the first full moon after the spring equinox. Thus the date would travel (a moveable feast), but could never be on Passover, which always begins on a full moon.
Both the Eastern and Western branches of the church followed this formula, even after the Eastern branch excommunicated the Roman Church in 1054. When Pope Gregory instituted the Gregorian calendar in 1582, the Eastern Orthodox Churches continued to follow the Julian calendar. Thus about a fourth of the time both Easters fall on the same day. Otherwise, there is a one, four, or five week difference.
But why is Passover so much later this year than Roman Easter? The Julian and Gregorian calendars are solar calendars. They base their dates on the sun. The Jewish calendar is a lunar-solar calendar, and primarily lunar. The months of the Jewish calendar begin with the full moon. “Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath.” (Col 2:16) Since a lunar month is just over 29 days, Jewish months are always 29 or 30 days long. That means that there are almost 12 ½ lunar months in a solar year. But Passover must always be in the spring. Left to itself, the Jewish calendar would have Passover earlier every year, eventually going through all four seasons. So every second or third year (in a pattern that coincides, incidentally with the major musical scale) has an additional leap-month, called Adar I, just before the month of Adar, which in such a year becomes Adar II. This ensures that Passover does not wander into winter.
This year happens to be a year in which the leap-month is added. Since Adar I was added to the Jewish calendar this year, Roman (Western) Easter actually occurred before Purim, almost a month before Passover.
Why, you may ask, is any of this important? I’ll tell you; I don’t know. Every indication from the earliest writings of the Christian church is that Christians celebrated the resurrection every Sunday. (Acts 20:7; 1 Cor 16:2) Thus “Easter” is an artificial celebration added many years later. Paul still celebrated Passover. (Acts 18:51; 1 Cor 5:8) Whether a Christian celebrates Passover or not is of small importance. What is important is that the final Passover lamb has been sacrificed, in the person of Jesus. That is what Christians celebrate. That is what we should celebrate every day of the year.