Saturday n.

Old English Sætern, name of the Roman god, also, in astronomy, the name of the most remote planet (then known); from Latin Saturnus, originally a name of an Italic god of agriculture, possibly from Etruscan. Derivation from Latin serere (past participle satus) "to sow" is said to be folk-etymology.

An ancient Italic deity, popularly believed to have appeared in Italy in the reign of Janus, and to have instructed the people in agriculture, gardening, etc., thus elevating them from barbarism to social order and civilization. His reign was sung by the poets as "the golden age." [Century Dictionary]

Identified with Greek Kronos, father of Zeus. Also the alchemical name for lead (late 14c.). In Akkadian, the planet was kaiamanu, literally "constant, enduring," hence Hebrew kiyyun, Arabic and Persian kaiwan "Saturn."

Old English dæg "period during which the sun is above the horizon," also "lifetime, definite time of existence," from Proto-Germanic *dages- "day" (source also of Old Saxon, Middle Dutch, Dutch dag, Old Frisian di, dei, Old High German tag, German Tag, Old Norse dagr, Gothic dags), according to Watkins, from PIE root *agh- "a day."  He adds that the Germanic initial d- is "of obscure origin." But Boutkan says it is from PIE root *dhegh- "to burn" (see fever). Not considered to be related to Latin dies (which is from PIE root *dyeu- "to shine").

Meaning originally, in English, "the daylight hours;" it expanded to mean "the 24-hour period" in late Anglo-Saxon times. The day formerly began at sunset, hence Old English Wodnesniht was what we would call "Tuesday night." Names of the weekdays were not regularly capitalized in English until 17c.

From late 12c. as "a time period as distinguished from other time periods." Day-by-day "daily" is from late 14c.; all day "all the time" is from late 14c.  Day off "day away from work" is attested from 1883; day-tripper first recorded 1897. The days in nowadays, etc. is a relic of the Old English and Middle English use of the adverbial genitive.

All in a day's work "something unusual taken as routine" is by 1820. The nostalgic those were the days is attested by 1907. That'll be the day, expressing mild doubt following some boast or claim, is by 1941. To call it a day "stop working" is by 1919; earlier call it a half-day (1838). One of these days "at some day in the near future" is from late 15c. One of those days "a day of misfortune" is by 1936.

Middle English sabat, from Old English sabat "seventh day of the week in the Jewish calendar; Saturday" as observed by the Jews as a day of rest from secular employment and of religious observance, from Old French sabat and directly from Latin sabbatum, from Greek sabbaton, from Hebrew shabbath, properly "day of rest," from shabath "he rested" (from labor). The spelling with -th is attested from late 14c. but was not widespread until 16c.

The Babylonians regarded seventh days as unlucky, and avoided certain activities on them; the Jewish observance might have begun as a similar custom. Among European Christians, the time of "Sabbath" shifted from the seventh day to the first (Sunday) via the Christians' celebration of the Lord's resurrection on the first day of the week (a Christian Sabbath) "though no definite law, either divine or ecclesiastical, directed the change" [Century Dictionary], but elaborate justifications have been made. In English Sabbath as "Sunday" is evident by early 15c. The sense change was completed among the English people generally during the Reformation.

The original use of the word 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. Gothic Sabbato, Sabbatus probably are directly from Greek.

The meaning "any day (or month or year) in which religious rest is enjoined" is by late 14c.; the word also was used in Medieval Latin of any feast day, the solstice, etc. Sabbath-breaking "act of profaning the Sabbath" is attested from 1650s (to break the Sabbath is from late 14c.), formerly a legal violation in parts of the old U.S., "immoral, disturbing, or unnecessary labors or practices" [Century Dictionary]. Sabbath-school is by 1798.