The festival of the Resurrection of the Lord (or Easter Sunday) is the center of the Christian year. On this occasion the church joyfully proclaims the good news that is at the very heart of the gospel: that Jesus Christ is risen from the dead.
It is sometimes said that every Sunday is a little Easter; liturgical theologian Laurence Stookey suggests that it might be more appropriate to say that every Easter is a great Sunday (Calendar: Christ’s Time for the Church, Abingdon Press, 1996, 158-161). Easter Sunday is the Lord’s Day writ large: a great annual celebration of Christ’s resurrection on the first day of the week. As such, the service should be centered around the typical and fundamental elements of Christian worship on the Lord’s Day: the proclamation of the Word and the celebration of the Lord’s Supper. Easter Sunday is also an especially appropriate time for the sacrament of Baptism, if not celebrated during the Easter Vigil of the previous night.
Easter Sunday is something like the keystone of an arch — the top and center stone upon which all the other stones lean and depend — both in terms of its theological significance and its relation to other events in the Christian year. Theologically speaking, the faith we claim and the life we live depend on the affirmation, celebration and proclamation of Christ’s resurrection. In a chronological sense — since Easter is a “moveable feast,” taking place on a different date each year — all the other events of the Christian year (from the Transfiguration of the Lord and Ash Wednesday through Pentecost and Trinity Sunday) pivot around the date of Easter Sunday, shifting accordingly.
For Western Christians (Catholics and Protestants) the date of Easter is the first Sunday that comes after the first full moon that occurs on or after March 21 (the Spring Equinox) — occasionally shifted to the following Sunday, when the original date happens to coincide with the Jewish Passover. This computation means that Easter always occurs sometime between March 22 and April 25, inclusive. The Eastern Churches (Greek and Russian Orthodox, e.g.) use a different set of astronomical tables based on the Julian Calendar (instead of the Gregorian), which means that Orthodox Easter generally follows the Western date by one, four, or five weeks (sometimes occurring in early May).