Technically speaking the season of Easter begins during the Easter Vigil on Holy Saturday, which starts after sundown.
Often Catholics will ask when Easter “officially” starts. Does Easter begin on Holy Saturday?
Holy Saturday is meant to be a day of prayer and silence, preparing our hearts for the celebration of Jesus’ resurrection on Easter morning.
For many centuries there was no liturgy on Holy Saturday, and the Easter Vigil only began after midnight.
Author Herbert Thurston gives a brief explanation in his book, Lent and Holy Week.
[T]he Mass which is now sung on Holy Saturday … was not originally a Mass for Holy Saturday at all, but coming at the end of the long ceremonies of the great vigil, was in reality the midnight Mass of Easter Sunday. Probably in the earliest stage of the celebration this point was not reached until long after midnight, when the day was already beginning to break. There was every reason then why the joyous exultation of the Resurrection should find its first expression there. Theoretically Holy Saturday, like Good Friday, was an ‘aliturgical’ day, a day without a Mass.
The Easter Vigil in the Roman Catholic Church now begins after sundown, starting with an elaborate liturgy of lighted candles and readings from the Bible.
Shortly before the Gospel is read, the “Alleluia” is intoned for the first time since before Lent. It is a liturgical way of celebrating Jesus’ resurrection.
For someone who wants to know the “official” start of the celebratory Easter season, it would be after that glorious singing of “Alleluia.”
Try to hold-off on any Easter celebrations until after the Easter Vigil, which for many people won’t end until 10:00 or 11:00 p.m.