What we call the Saturday evening Mass is technically a “vigil Mass.” Vigil comes from the Latin word to “keep watch.” The prayer candles in church are known as “vigil lights” for the same reason: They symbolically keep watch over our prayer intentions. Christians are “vigilant”—always watching—for the Day of the Lord, a reminder that every Sunday of the year is a little Easter, as the early Fathers of the Church noted.
Holy Saturday night inaugurates “the mother of all vigils” at nightfall with the lighting of the Easter fire and the sharing of the light of Christ from the paschal candle. Darkness is a necessary component of the Easter Vigil because the coming of Christ’s light dispels it. Vigils sprang up before many great feasts of the church, including Christmas, Epiphany, Ascension, Pentecost, the Immaculate Conception, the Assumption, and the eight feasts of the apostles, Saint John the Baptist, All Saints, and, curiously, the feast of Saint Lawrence.
The weekly vigil Mass accompanied the liturgical reforms of Vatican II, acknowledging the pastoral needs of workers and others who cannot keep the Sunday obligation. Pope John Paul II noted in 1998: “Because the faithful are obliged to attend Mass unless there is a grave impediment, pastors have the corresponding duty to offer to everyone the real possibility of fulfilling the precept. . . . From a liturgical point of view, in fact, holy days begin with First Vespers. Consequently, the liturgy of what is sometimes called the ‘Vigil Mass’ is in effect the ‘festive’ Mass of Sunday.”
Many ancient cultures perceived a “day” as lasting from sundown to sundown, including our Jewish ancestors. This perspective is recognized in the Creation story, in which “evening came, and morning followed—the first day.” Perhaps from the desire to distinguish between the Hebrew Sabbath on Saturday and the Lord’s Day on Sunday, the Hebrew day is not mentioned in the Catechism of the Catholic Church, which notes only the canonical permission to celebrate the vigil Mass (CCC 2180). The permission itself can be found in the 1983 Code of Canon Law, which states that “the precept of participating in the Mass is satisfied by assistance at a Mass which is celebrated anywhere in a Catholic rite either on the holy day or on the evening of the preceding day” (canon 1248). This canon gives the most direct answer to “why” by legitimating the practice. The rationale of “why” is justified by the primacy of the Easter Vigil in our liturgical life.
• Genesis 1:5, 8, 13, 19, 23, 31
• Dies Domini, "On Keeping the Lord's Day Holy," Apostolic Letter of Pope John Paul II, 1998
• The Lord's Day: Reflections on Dies Domini by Bill Huebsch (Twenty-Third Publications)