"day given to pastime or diversion, a day exempt from work," c. 1600, from play (v.) + day.

mid-15c., "day that begins a quarter of the year," designated as days when rents were paid and contracts and leases began or expired, from quarter (n.1). They were, in England, Lady day (March 25), Midsummer day (June 24), Michaelmas day (Sept. 29), and Christmas day (Dec. 25); in Scotland, keeping closer to the pre-Christian Celtic calendar, they were Candlemas (Feb. 2), Whitsunday (May 15), Lammas (Aug. 1), and Martinmas (Nov. 11). Quarter in the sense "period of three months; one of the four divisions of a year" is recorded from late 14c. Related: Quarter days.

1918, "date set for the beginning of a military operation," with D as an abbreviation of day; compare H-hour, also from the same military order of Sept. 7, 1918:

The First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of the St. Mihiel salient. [Field Order No. 8, First Army, A.E.F.]

"They designate the day and hour of the operation when the day and hour have not yet been determined, or where secrecy is essential" [U.S. Army Center of Military History Web site]. Now almost exclusively of June 6, 1944.

the spelling used in the U.S. congressional resolution first recognizing it, May 9, 1908.

1747, originally a day of military exercise and review (see field (v.)); figurative sense "any day of unusual bustle, exertion, or display" [Century Dictionary] is from 1827.

initialism (acronym) for Victory in Europe, from September 1944 (see victory).

"belonging to recent times," 1842; see latter (adj.). Originally in Latter-day Saints, the Mormon designation for themselves.

also daybook, "book for recording events and transactions of the day," 1570s, from day (n.) + book (n.).

masculine or positive principle in Chinese philosophy, 1670s, from Mandarin yang, said to mean "male, daylight, solar," or "sun, positive, male genitals."

1550s, "a number attached to a year to show the number of days into the calendar moon on which the solar year begins;" 1580s, "number of days by which the solar year exceeds a lunar one of 12 moons;" from French épacte (12c.), from Late Latin epacta "an intercalary day," from Greek epakte (plural epaktai, in epaktai hemerai "intercalary days"), from fem. of epaktos "brought in, imported, alien," verbal adjective of epagein "to add, bring forward," also "intercalate," from epi "on" (see epi-) + agein "put in motion, move; push forward, advance," from PIE root *ag- "to drive, draw out or forth, move." Related: Epactal.