In the biblical notion of time, one day ends and another begins at sundown, rather than at 12:00 midnight. (Recall that “evening came, and morning followed – the first [etc.] day” in the creation story of Genesis 1:1-2:4a.) Thus when evening falls on Saturday, according to the understanding of the Church, Sunday begins. When you attend a Saturday evening mass, you hear the lectionary readings and prayers for Sunday; participating in this mass “counts” to fulfill your Sunday obligation. Anticipating the Sunday mass – celebrating it on Saturday evening – came about in the latter half of the 20th century, as the rhythms of modern life made attending Sunday morning mass increasingly difficult for some. In 1953 Pope Pius XII allowed local ordinaries to permit the celebration of mass in the evening in their territories; in the 1983 revision of the Code of Canon Law, this permission was granted universally.
However, we go to mass not merely to fulfill our obligation but to gather with our brothers and sisters in Christ, to pray and to be transformed by the liturgy. The Saturday evening mass should be celebrated with as much attention and dignity as any Sunday mass. Slipping in after the mass has started or sneaking out before it has finished fulfills the letter of the law but not its spirit, regardless of what day it is. More important than when you go to mass is how you go. Choose the time that best allows you to participate fully, actively and consciously in the Eucharistic sacrifice. Also, the commandment to keep the Lord’s day holy still holds, even if you must meet work or family obligations on Sunday. Attending mass is not the only way to keep a day holy. If you do not go to mass on Sunday, make sure that you live that day in a prayerful awareness that it is a day dedicated to the Lord.