Easter Saturday is the final day of Lent, of Holy Week, and of the Easter Triduum, the three days Maundy Thursday, Good Friday, and Holy Saturday. The day is also known as the Easter Vigil, a name more properly applied to the Mass on Easter Saturday night, has had a long and varied history.
As the Catholic Encyclopaedia notes, "in the early Church this was the only Saturday on which fasting was permitted. Fasting is a sign of penance, but on Good Friday, Christ paid with His own Blood the debt of our sins. Thus, for many centuries, Christians regarded both Saturday and Sunday, the day of Christ's Resurrection, as days on which fasting was forbidden. (That practice is still reflected in the Lenten disciplines of the Eastern Catholic and Eastern Orthodox Churches, which lighten their fasts slightly on Saturdays and Sundays.)
By the second century, Christians had begun to observe a total fast (no food of any kind) for 40 hours before Easter, which meant that the entire day of Easter Saturday was a day of fasting. As on Good Friday, there is no Mass offered for Holy Saturday. The Easter Vigil Mass, which takes place after sundown on Easter Saturday, properly belongs to Easter Sunday, since liturgically, each day begins at sundown on the previous day. Unlike on Good Friday, when Holy Communion is distributed at the afternoon liturgy commemorating Christ's Passion, on Holy Saturday the Eucharist is only given to the faithful as viaticum—that is, only to those in danger of death, to prepare their souls.
In the early Church, Christians gathered on the afternoon of Easter Saturday to pray and to confer the Sacrament of Baptism on catechumens—converts to Christianity who had spent Lent preparing to be received into the Church. This vigil lasted through the night until dawn on Easter Sunday, when the Alleluia was sung for the first time since the beginning of Lent, and the faithful—including the newly baptised—broke their 40-hour fast by receiving Communion.
In the Middle Ages, beginning roughly in the eighth century, the ceremonies of the Easter Vigil, especially the blessing of new fire and the lighting of the Easter candle, began to be performed earlier and earlier. Eventually, these ceremonies were performed on Holy Saturday morning. The whole of Holy Saturday, originally a day of mourning for the crucified Christ and of expectation of His Resurrection, now became little more than an anticipation of the Easter Vigil.
With the reform of the liturgies for Holy Week in 1956, those ceremonies were returned to the Easter Vigil itself (that is, to the Mass celebrated after sundown on Holy Saturday), and thus the original character of Easter Saturday was restored.