Sabbatarian (n.)youdaoicibaDictYouDict
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; earlier sabbatary (1590s). Related: Sabbatartianism.
Sabbath (n.)youdaoicibaDictYouDict
Old English sabat "Saturday as a day of rest," as observed by the Jews, from Latin sabbatum, from Greek sabbaton, from Hebrew shabbath, properly "day of rest," from shabath "he rested." Spelling with -th attested from late 14c., not widespread until 16c.

The Babylonians regarded seventh days as unlucky, and avoided certain activities then; the Jewish observance might have begun as a similar custom. Among European Christians, from the seventh day of the week it began to be applied early 15c. to the first day (Sunday), "though no definite law, either divine or ecclesiastical, directed the change" [Century Dictionary], but elaborate justifications have been made. The change was driven by Christians' celebration of the Lord's resurrection on the first day of the week, a change completed during the Reformation.

The original meaning is preserved in Spanish Sabado, Italian Sabbato, and other languages' names for "Saturday." Hungarian szombat, Rumanian simbata, French samedi, German Samstag "Saturday" are from Vulgar Latin sambatum, from Greek *sambaton, a vulgar nasalized variant of sabbaton. Sabbath-breaking attested from 1650s.
Saturday (n.)youdaoicibaDictYouDict
seventh day of the week, Old English sæterdæg, sæternesdæg, literally "day of the planet Saturn," from Sæternes (genitive of Sætern; see Saturn) + Old English dæg (see day). Partial loan-translation of Latin Saturni dies "Saturn's day" (compare Dutch Zaterdag, Old Frisian Saterdi, Middle Low German Satersdach; Irish dia Sathuirn, Welsh dydd Sadwrn). The Latin word itself is a loan-translation of Greek kronou hemera, literally "the day of Cronus."

Unlike other English day names, no god substitution seems to have been attempted, perhaps because the northern European pantheon lacks a clear corresponding figure to Roman Saturn. A homely ancient Nordic custom, however, seems to be preserved in Old Norse laugardagr, Danish lørdag, Swedish lördag "Saturday," literally "bath day" (Old Norse laug "bath").

German Samstag (Old High German sambaztag) appears to be from a Greek *sambaton, a nasalized colloquial form of sabbaton "sabbath," also attested in Old Church Slavonic sabota, Polish sobota, Russian subbota, Hungarian szombat, French samedi.

Saturday night has been used figuratively to suggest "drunkenness and looseness in relations between the young men and young women" since at least mid-19c. Saturday-night special "cheap, low-caliber handgun" is American English, attested from 1976 (earlier Saturday-night pistol, 1929).