The dates for Passover are based on the Jewish calendar. What many people don't know is that the Jewish liturgical year was also originally the basis for the dates for Easter.
There are two dates each year for the Christian observance of Easter---one for the Western Christian sects and one for Orthodox. The Council of Nicaea in A.D. 325 determined how both dates are now calculated.
The dates of these religious holidays vary from year to year, depending on astronomical movements and/or the ecclesiastical calendar.
Passover, which celebrates the exodus of the Hebrew tribes from slavery in Egypt, took place just before the crucifixion and resurrection of Christ, and the two holidays have been linked by proximity and symbolism ever since. Pasch, a word that means Passover, is also used for Easter.
Determine the dates of Passover, referring to a Jewish Calendar.
Passover begins on the 15th day of Nissan, the seventh month in the Jewish calendar (late March to early April). Hebrew days begin and end at sundown, so the holiday would actually begin at sundown on the preceding day.
Passover extends seven days for Israelis and Reform Jews, eight days for others around the globe. It culminates on the 21st day of Nissan in Israel and for Reform Jews, and on the 22nd of Nissan elsewhere.
Determine the date of Easter for Western Christians, referring to the Gregorian calendar. Easter falls on the first Sunday that follows the full moon that occurs on or after the spring equinox. Western Christians calculate this using March 21st as a fixed date for the equinox. Instead of an astronomical full moon, they use the date of the ecclesiastical full moon, or the 14th day of a lunar month, determined by tables created by the church.
Determine the Easter date for Orthodox Christians, using the Julian calendar. Easter falls on the first Sunday that follows the full moon that occurs on or after the spring equinox. Orthodox Christians calculate this in accordance with the actual, astronomical full moon and the equinox as observed in Jerusalem.