Origin and history of today

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." From day to day was in late Old English; 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.

Old English to, ta, te, "in the direction of, as far as (a place, state, goal)," opposite of from; also "for the purpose of, furthermore;" from West Germanic *to (source also of Old Saxon and Old Frisian to, Dutch toe, Old High German zuo, German zu "to"). Not found in Scandinavian, where the equivalent of till (prep.) is used.

This is reconstructed to be from PIE pronominal base *do- "to, toward, upward" (source also of Latin donec "as long as," Old Church Slavonic do "as far as, to," Greek suffix -dē "to, toward," Old Irish do, Lithuanian da-), from demonstrative *de-. Also see too.

English to also supplies the place of the dative in other languages. The near-universal use of to as the verbal particle with infinitives (to sleep, to dream, etc.) arose in Middle English out of the Old English dative use of to and helped shade out the Old English inflectional endings. In this use to is a mere sign, without meaning. Compare similar use of German zu, French à, de.

As an adverb of motion, direction, etc., "to a place in view, to a thing to be done," in Old English. This use was frequent in Middle English in verbal combinations where it renders Latin ad-, com-, con-, ex-, in-, ob-. As a conjunction, "until, up to the time that," by late Old English.

The distribution of verbs among at, to, with, of has been idiosyncratic and varied. Before vowels it was sometimes shortened to t'. The phrase what's it to you "how does that concern you?" (1819) is a modern form of an old question:

Huæd is ðec ðæs?

[John xxi:22, in Lindisfarne Gospel, c.950]

Used absolutely at the end of a clause. with ellipsis of infinitive (same as the proceeding clause: would do it but don't have time to), it is attested from 14c.; OED reports it "rare before 19th c.; now a frequent colloquialism."