Sabbatarianism is the fundamental doctrine of those Christians (sabbatarians) who believe that Saturday is the Christian Sabbath, and that it should be devoutly observed in accordance with the Fourth Commandment of the Law, which forbids work on the Sabbath because it is a holy day. See Exodus 20:8.
The Seventh Day Adventists are the largest and most well-known of these groups (though a sizeable portion of Christians consider them to be a cult). There are also smaller groups such as Seventh Day Baptists.
Although it is common for megachurches to hold services on Saturdays (as an option, usually to accommodate the large crowds attending every weekend), virtually no megachurch holds to sabbatarian views.
- Early Church Fathers - Worship on Sabbath or Sunday (freerepublic.com)
- The Early Church Fathers and the Sabbath (ukapologetics.net)
- Could you tell me why the Sunday-law of Constantine is ignored as if Sunday is mentioned in the Greek Scriptures. Is this not a turning point of the history of the apostasy of the church as prophesied in 2nd Thessalonians 2? (churchhistory101.com)
- The Sabbath (christian-history.org)
- Sabbath to Sunday: What Really Happened Under Constantine (christian-history.org)