The strict observance of Sunday as a day of rest (i.e. as a day for church-going, devotional reading and moral fortification), in accordance with the Fourth of the Ten Commandments. The Christian approximation of the Jewish Sabbath.
Sabbatarianism was a characteristic of the Reformation in England and Scotland, strictly enforced by Puritan legislation in the 17th century. When the monarchy was restored under Charles II, the Sabbatarian requirements were relaxed somewhat, only to be revived by Evangelicals in the late 18th century. This culminated in the Lord's Day Observance Act of 1781.
In the 19th and early 20th centuries, Sabbatarianism enjoyed popular support in the middle classes of Great Britain, who disdained the "degenerate" custom of a "Continental Sunday" (i.e. lax observance of the Sabbath), and viewed any attempt to slacken the Sabbatarian rules as secularist subversion.