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What does "D" stand for in the name D-Day?

In the English-speaking world, D-Day is the key date of a larger, usually military, company. Tag "X" is a comparable German term. The abbreviation D-Day (just like H-Hour) has no specific meaning, except that it refers to the time of a major military operation. "D-Day" is also an abbreviation for Decision Day, Delivery Day, Deliverance Day, Doomsday or Debarkation Day, but this origin of the term is not certain.

As an alternative explanation, one often finds that D and H are simply repetitions of day and hour, respectively. The first proven use of the term can be found in the "Field Order Number 9, First Army, American Expeditionary Forces" of September 7, 1918 during World War I. There it says: "The First Army will attack at H hour on D day with the object of forcing the evacuation of the St. Mihiel Salient."

Presumably, this was initially only used as a placeholder for a certain day and time, although these have not yet been determined.

The quote can be found in the volume "Combat Orders" published in 1922 by the "General Service Schools" in Fort Leavenworth, Kansas. For the first time, "D-Day" was used as a code for the day of a military operation in World War I. This D-Day was the assault by American troops on the German-occupied Lorraine city of Saint-Mihiel in September 1918.

In particular, the term D-Day is known today for June 6, 1944, when the landing of Allied troops in Normandy began.

The question was answered with the help of Wikipedia, the free encyclopedia on the Internet.