The word Sabbath is derived from various words in Hebrew, Greek, and Latin, all of them meaning "to rest." Sabbath is the seventh day of the week (Saturday), named in the Ten Commandments of the Old Testament as a day of worship and rest and thus observed by Jews and some Christian sects. Sabbath is also a term for the first day of the week, Sunday, observed by most Christian churches as a day of rest. Sunday is derived from an Old English word meaning "day of the sun."
Dictionary of problem words and expressions. Harry Shaw. 1975.