Paschal Vigil

Easter Vigil, also called the Paschal Vigil or the Great Vigil of Easter, is a service held in traditional Christian churches as the first official celebration of the Resurrection of Jesus. Historically, it is during this service that people are baptized and that adult catechumens are received into full communion with the Church. It is held in the hours of darkness between sunset on Holy Saturday and sunrise on Easter Day – most commonly in the evening of Holy Saturday or midnight – and is the first celebration of Easter, days traditionally being considered to begin at sunset.

The Roman Missal states: "Of this night’s Vigil, which is the greatest and most noble of all solemnities, there is to be only one celebration in each church. It is arranged, moreover, in such a way that after the Lucernarium and Easter Proclamation (which constitutes the first part of this Vigil), Holy Church meditates on the wonders the Lord God has done for his people from the beginning, trusting in his word and promise (the second part, that is, the Liturgy of the Word) until, as day approaches, with new members reborn in Baptism (the third part), the Church is called to the table the Lord has prepared for his people, the memorial of his Death and Resurrection until he comes again (the fourth part)."

Among liturgical western churches including the Roman Catholic Church, the Anglican Communion, and Lutheran churches, the Easter Vigil is the most important service of public worship and Masses of the liturgical year, marked by the first use since the beginning of Lent of the exclamatory "Alleluia", a distinctive feature of the Easter season.

...
Wikipedia