What is the X in X-mas?
Old English dæg "day," also "lifetime," from Proto-Germanic *dagaz (cf. Old Saxon, Middle Dutch, Dutch dag, Old Frisian dei, Old High German tag, German Tag, Old Norse dagr, Gothic dags), from PIE *dhegh-.
Not considered to be related to Latin dies (see diurnal), but rather to Sanskrit dah "to burn," Lithuanian dagas "hot season," Old Prussian dagis "summer." Meaning originally, in English, "the daylight hours;" expanded to mean "the 24-hour period" in late Anglo-Saxon times. Day off first recorded 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.
1918, "date set for the beginning of a military operation," with D as an abbreviation of day, cf. 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.
See under sidereal time, solar day.
The code name for the first day of a military attack, especially the American and British invasion of German-occupied France during World War II on June 6, 1944 (see invasion of Normandy). This marked the beginning of the victory of the Allies in Europe. Germany surrendered less than a year later.
The Jews reckoned the day from sunset to sunset (Lev. 23:32). It was originally divided into three parts (Ps. 55:17). "The heat of the day" (1 Sam. 11:11; Neh. 7:3) was at our nine o'clock, and "the cool of the day" just before sunset (Gen. 3:8). Before the Captivity the Jews divided the night into three watches, (1) from sunset to midnight (Lam. 2:19); (2) from midnight till the cock-crowing (Judg. 7:19); and (3) from the cock-crowing till sunrise (Ex. 14:24). In the New Testament the division of the Greeks and Romans into four watches was adopted (Mark 13:35). (See WATCHES.) The division of the day by hours is first mentioned in Dan. 3:6, 15; 4:19; 5:5. This mode of reckoning was borrowed from the Chaldeans. The reckoning of twelve hours was from sunrise to sunset, and accordingly the hours were of variable length (John 11:9). The word "day" sometimes signifies an indefinite time (Gen. 2:4; Isa. 22:5; Heb. 3:8, etc.). In Job 3:1 it denotes a birthday, and in Isa. 2:12, Acts 17:31, and 2 Tim. 1:18, the great day of final judgment.