Catechism of the Catholic Church

1168 Beginning with the Easter Triduum as its source of light, the new age of the Resurrection fills the whole liturgical year with its brilliance. Gradually, on either side of this source, the year is transfigured by the liturgy. It really is a "year of the Lord's favor." ⁴² The economy of salvation is at work within the framework of time, but since its fulfillment in the Passover of Jesus and the outpouring of the Holy Spirit, the culmination of history is anticipated "as a foretaste," and the kingdom of God enters into our time.

1169 Therefore Easter is not simply one feast among others, but the "Feast of feasts," the "Solemnity of solemnities," just as the Eucharist is the "Sacrament of sacraments" (the Great Sacrament). St. Athanasius calls Easter "the Great Sunday" ⁴³ and the Eastern Churches call Holy Week "the Great Week." The mystery of the Resurrection, in which Christ crushed death, permeates with its powerful energy our old time, until all is subjected to him.

1170 At the Council of Nicaea in 325, all the Churches agreed that Easter, the Christian Passover, should be celebrated on the Sunday following the first full moon (14 Nisan) after the vernal equinox. Because of different methods of calculating the 14th day of the month of Nisan, the date of Easter in the Western and Eastern Churches is not always the same. For this reason, the Churches are currently seeking an agreement in order once again to celebrate the day of the Lord's Resurrection on a common date.

1171 In the liturgical year the various aspects of the one Paschal mystery unfold. This is also the case with the cycle of feasts surrounding the mystery of the incarnation (Annunciation, Christmas, Epiphany). They commemorate the beginning of our salvation and communicate to us the first fruits of the Paschal mystery.