The Easter Vigil is officially the first service of Easter. In fact, Christian feast days generally begin at sunset on the previous day (best known in the example of Christmas Eve). For this reason, the duration of the Easter Triduum (“three days”) is from the evening of Maundy Thursday to the evening of Easter Sunday. The same principle applies to the Jewish reckoning of liturgical time, in which the sabbath begins at dusk and continues to nightfall of the following day. This is reflected in the priestly “refrain” of the Genesis 1 creation story: “and it was evening, and it was morning, the nth day.”

The Easter Vigil has four movements: (1) the Service of Light, a celebration of the light of Christ at which a new fire is kindled from which the Paschal Candle is lit, (2) the Service of Readings, which includes as many as nine readings from the grand story of salvation of the Old and New Testaments interspersed with psalms and canticles, silence and prayers, (3) the Service of Baptism, the primary annual occasion for baptisms (particularly in the early church) and a time for the reaffirmation of the baptismal covenant and (4) the Service of the Eucharist, a joyous feast in the presence of the risen Christ and an anticipation of the eschatological banquet of the realm of God.

Excerpt taken from the Presbyterian Church USA website.