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 day is celebrated only in preparation for Easter. It is the centre of the greater part of the ecclesiastical year. The order of Sundays from Septuagesima to the last Sunday after Pentecost, the feast of the Ascension, Pentecost, Corpus Christi, and all other movable feasts, from that of the Prayer of Jesus in the Garden (Tuesday after Septuagesima) to the feast of the Sacred Heart (Friday after the octave of Corpus Christi), depend upon the Easter date. 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.