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 Sabato, 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.