"Easter is the central celebration of the Christian liturgical year. It is the oldest and most important christian feast, celebrating the Resurrection of Jesus Christ. The date of Easter determines the dates of all movable feast except those of the Advent. Easter occurs on the Sunday after the first formal new moon on or after the official spring equinox on March 21; it can occur between the Gregorian calendar dates of March 22 and April 25, inclusive , for Western churches and between the Gregorian calendar dates of April 4 and May 8, inclusive, for Eastern churches that use the Julian calendar for the dating of Easter."