Seventh-day Adventists observe the Sabbath from Friday sundown to Saturday sundown, following the biblical day's evening-to-evening reckoning, setting aside the period for worship, rest and family time and generally avoiding ordinary work, business and non-essential activity across those twenty-four hours. Sabbath school, a structured period of Bible study preceding the main worship service, is a distinctive feature of Adventist Saturday observance found across the denomination worldwide.
The practice sets Adventists apart from the great majority of Christian bodies, which moved their principal day of worship to Sunday in commemoration of Christ's resurrection sometime in the first two centuries of the church, a shift Adventists regard as an unauthorized human tradition rather than a scriptural command, and one they trace historically to Roman imperial influence following Constantine's Sunday-rest legislation of 321.
Because the underlying evangelical theology of salvation by grace through faith is otherwise close to that of other conservative Protestant bodies, Sabbath observance functions as Adventism's clearest and most publicly visible marker, the practice most likely to be named first by both Adventists and outside observers describing what distinguishes the denomination from its Protestant neighbors.