April 19, 2011 to April 26, 2011:
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.
April 24, 2011:
Easter, celebrating Jesus Christ's rising from the dead, 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 A.D. established that Easter would be celebrated on Sundays; before that Easter was celebrated on different days in different places in the same year.