The Jewish liturgical year is not simply the basis for Jewish holidays, but for the Christian movable feasts as well—those annual holidays that do not fall on a fixed date but vary according to astronomical occurrences.
The celebration of Passover took place just before the Crucifixion and Resurrection of Christ, and the two holidays have been entwined from the beginning—the word Pasch, originally meaning Passover, came to mean Easter as well.
Passover, or Pesach in Hebrew, the holiday commemorating the Hebrews' exodus from slavery in Egypt, lasts seven days in Israel and among Reform Jews, and eight days elsewhere around the world. It begins on the 15th day of Nisan, which is the seventh month in the Jewish calendar. It ends on the 21st of Nisan in Israel (and for Reform Jews) and on the 22nd of Nisan elsewhere. Since Hebrew days begin and end at sundown, Passover begins at sundown on the preceding day.
See also dates of other Jewish feasts.
Easter - (Western Churches)
Easter is calculated as the first Sunday after the paschal full moon that occurs on or after the vernal equinox. If the full moon falls on a Sunday, then Easter is the following Sunday. The holiday can occur anywhere between March 22 and April 25.
The Western church does not use the actual, or astronomically correct date for the vernal equinox, but a fixed date (March 21). And by full moon it does not mean the astronomical full moon but the "ecclesiastical moon," which is based on tables created by the church. These constructs allow the date of Easter to be calculated in advance rather than determined by actual astronomical observances, which are naturally less predictable. See also A Tale of Two Easters.
The Council of Nicaea in 325 established that Easter would be celebrated on Sundays; before that Easter was celebrated on different days in different places in the same year. See also dates of other Christian movable feasts.
Easter - (Orthodox Church)
The Orthodox church uses the same formula to calculate Easter, but bases the date on a slightly different calendar—the Julian calendar instead of the more contemporary Gregorian one, the calendar that is most widely used today. Consequently, both churches only occasionally celebrate Easter on the same day.
Unlike the Western Church, the Eastern Church sets the date of Easter according to the actual, astronomical full moon and the actual equinox as observed along the meridian of Jerusalem, site of the Crucifixion and Resurrection. See also A Tale of Two Easters and dates of other Orthodox movable feasts.