Easter, the Sunday of the Resurrection, Pascha, or Resurrection Day, is the most important religious feast of the Christian liturgical year, observed at some point between late March and late April each year (early April to early May in Eastern Christianity), following the cycle of the moon. It celebrates the resurrection of Jesus, which occurred on the third day of his death by crucifixion. Easter also refers to the season of the church year, called Eastertide or the Easter Season, lasting for fifty days, from Easter Sunday to Pentecost.
The Great Vigil, when observed, is the first service of Easter Day. It is celebrated at a convenient time between sunset on Holy Saturday and sunrise on Easter Morning. After 40 days of Lent, we get to say "Alleluia!" again!
Wikipedia article on Easter
"Easter Customs" at Full Homely Divinity