late 14c., "daily, happening every day," from Late Latin diurnalis "daily," from Latin dies "day" + -urnus, an adjectival suffix denoting time (compare hibernus "wintery"). Dies "day" is from PIE root *dyeu- "to shine" (source also of Sanskrit diva "by day," Welsh diw, Breton deiz "day;" Armenian tiw; Lithuanian diena; Old Church Slavonic dini, Polish dzień, Russian den).
From early 15c. as "performed in or occupying one day;" 1620s as "of or belonging to the daytime (as distinguished from nocturnal). Related: Diurnally.
late 14c. in reference to the Roman calendar, "ninth day (by inclusive reckoning) before the ides of each month" (7th of March, May, July, October, 5th of other months), from Old French nones and directly from Latin nonæ (accusative nonas), fem. plural of nonus "ninth," contracted from *novenos, from novem "nine" (see nine).
The ecclesiastical sense of "daily office said originally at the ninth hour of the day" is from 1709; originally fixed at ninth hour from sunrise, hence about 3 p.m. (now usually somewhat earlier), from Latin nona (hora) "ninth (hour)," from fem. plural of nonus "ninth." It was also used from c. 1300 in a general sense of "midday" (see noon). For for the nones, see nonce.
Similar constructions exist in other Germanic languages, such as Dutch van daag "from-day," Danish and Swedish i dag "in day." German heute is from Old High German hiutu, from Proto-Germanic *hiu tagu "on (this) day," with first element from PIE pronominal stem *ki-, represented by Latin cis "on this side."
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.
c. 1600, from Italian Decamerone, titleof Boccaccio's 14c. collection of 100 tales supposedly told over 10 days, from Greek deka "ten" (from PIE root *dekm- "ten") + hēmera "day," from PIE *Hehmer "day" (source also of Armenian awr "day"). Related: Decameronic.