first day of the week; in Christianity, the Lord's Day, the weekly memorial of Jesus Christ's Resurrection from the dead. The practice of Christians gathering together for worship on Sunday dates back to apostolic times, but details of the actual development of the custom are not clear. Before the end of the 1st century AD, the author of Revelation gave the first day its name of the “Lord's Day” (Rev. 1:10). Saint Justin Martyr (Justin Martyr, Saint) (c. 100–c. 165), philosopher and defender of the Christian faith, in his writings described the Christians gathered together for worship on the Lord's Day: the gospels or the Old Testament was read, the presiding minister preached a sermon, and the group prayed together and celebrated the Lord's Supper.

The emperor Constantine (Constantine I) (d. 337), a convert to Christianity, introduced the first civil legislation concerning Sunday in 321, when he decreed that all work should cease on Sunday, except that farmers could work if necessary. This law, aimed at providing time for worship, was followed later in the same century and in subsequent centuries by further restrictions on Sunday activities. See also Sabbatarianism; week.