Words related to day
c. 1300 (as two words from mid-12c., daies liht), "the light of day," from day + light (n.); its figurative sense of "clearly visible open space between two things" (1820) has been used in references to boats in a race, U.S. football running backs avoiding opposing tackles, a rider and a saddle, and the rim of a glass and the surface of the liquor. The (living) daylights that you beat or scare out of someone were originally slang for "the eyes" (1752), extended figuratively to the vital senses. Daylight-saving is attested by 1908.
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.
In medieval England doomsday was expected when the world's age had reached 6,000 years from the creation, which was thought to have been in 5200 B.C.E. Bede, c. 720, complained of being pestered by rustici asking him how many years till the sixth millennium ended. However there is no evidence for the story of a general panic in Christian Europe in the year 1000 C.E.
Doomsday machine as the name of a hypothetical nuclear bomb powerful enough to wipe out human life (or all life) on earth is from 1960.