"at some indefinite future date," 1768, from some + day. As two words, in the same sense, from late 14c.

MISS SOMEDAY.

Poor Charley wooed, but wooed in vain,

From Monday until Sunday;

Still Cupid whisper'd to the swain

"You'll conquer Betsey Someday."

[The Port Folio, June 1816]

first day of the week, Old English sunnandæg (Northumbrian sunnadæg), literally "day of the sun," from sunnan, oblique case of sunne "sun" (see sun (n.)) + dæg "day" (see day). A Germanic loan-translation of Latin dies solis "day of the sun," which is itself a loan-translation of Greek hēmera heliou. Compare Old Saxon sunnun dag, Old Frisian sunnandei, Old Norse sunnundagr, Dutch zondag, German Sonntag "Sunday."

In European Christian cultures outside Germanic often with a name meaning "the Lord's Day" (Latin Dominica). Sunday-school dates from 1783 (originally for secular instruction); Sunday clothes is from 1640s. Sunday driver is from 1925.

third day of the week, Old English tiwesdæg, from Tiwes, genitive of Tiw "Tiu," from Proto-Germanic *Tiwaz "god of the sky," the original supreme deity of ancient Germanic mythology, differentiated specifically as Tiu, ancient Germanic god of war, from PIE *deiwos "god," from root *dyeu- "to shine," in derivatives "sky, heaven, god." Cognate with Old Frisian tiesdei, Old Norse tysdagr, Swedish tisdag, Old High German ziestag.

The day name (second element dæg, see day) is a translation of Latin dies Martis (source of Italian martedi, French Mardi) "Day of Mars," from the Roman god of war, who was identified with Germanic Tiw (though etymologically Tiw is related to Zeus), itself a loan-translation of Greek Areos hēmera. In cognate German Dienstag and Dutch Dinsdag, the first element would appear to be Germanic ding, þing "public assembly," but it is now thought to be from Thinxus, one of the names of the war-god in Latin inscriptions.

Old English weorcdæg, from work (n.) + day (n.), but this apparently meant "day when work was suspended," as it glosses Latin feria. The modern word is perhaps a Middle English re-formation. As an adjective (c. 1500) it has generally only the literal sense (compare workaday).