Easter Vigil, also called the Paschal Vigil or the Great Vigil of Easter, is 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.
Source: Wikipedia.