One who observes Saturday as the Sabbath (as in Judaism)

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1610s, "a Christian unusually strict about Sabbath observation," from Latin Sabbatarius (adj.), from Sabbatum (see Sabbath ). Meaning "member of a Christian sect which maintained the Sabbath should be observed on the seventh day" is attested from 1640s; ...

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Sabbatarian \Sab`ba*ta"ri*an\, a. Of or pertaining to the Sabbath, or the tenets of Sabbatarians.

Wiktionary Word definitions in Wiktionary
a. Of or pertaining to the Sabbath, or the tenets of Sabbatarians. n. 1 A person who regards and keeps the seventh day of the week ("Saturday", the Israelite or Jewish Sabbath) as holy in conformity with the fourth commandment of the Decalogue, such as ...