Easter is the principal feast of the ecclesiastical year. Leo I (Sermo xlvii in Exodum) calls it the greatest feast (festum festorum), and says that Christmas is celebrated only in preparation for Easter. It is the centre of the greater part of the ecclesiastical year.
To have a correct idea of the Easter celebration and its Masses, we must remember that it was intimately connected with the solemn rite of baptism. The preparatory liturgical acts commenced on the eve and were continued during the night. When the number of persons to be baptized was great, the sacramental ceremonies and the Easter celebration were united. This connection was severed at a time when, the discipline having changed, even the recollection of the old traditions was lost. The greater part of the ceremonies was transferred to the morning hours of Holy Saturday.
Commemorating the slaying of the true Lamb of God and the Resurrection of Christ, the corner-stone upon which faith is built, it is also the oldest feast of the Christian Church, as old as Christianity, the connecting link between the Old and New Testaments.
The connection between the Jewish Passover and the Christian feast of Easter is real and ideal. Real, since Christ died on the first Jewish Easter Day; ideal, like the relation between type and reality, because Christ's death and Resurrection had its figures and types in the Old Law, particularly in the paschal lamb, which was eaten towards evening of the 14th of Nisan. In fact, the Jewish feast was taken over into the Christian Easter celebration; the liturgy (Exsultet) sings of the passing of Israel through the Red Sea, the paschal lamb, the column of fire, etc.
The connection between the Jewish and the Christian Pasch explains the movable character of this feast. Easter has no fixed date, like Christmas, because the 15th of Nisan of the Semitic calendar was shifting from date to date on the Julian calendar. Since Christ, the true Paschal Lamb, had been slain on the very day when the Jews, in celebration of their Passover, immolated the figurative lamb, the Jewish Christians in the Orient followed the Jewish method, and commemorated the death of Christ on the 15th of Nisan and His Resurrection on the 17th of Nisan, no matter on what day of the week they fell. For this observance they claimed the authority of St. John and St. Philip.