6 June 2019
American troops landing at Omaha Beach, Normandy, 6 June 1944. Photo by Herman V. Wall, licensed under Creative Commons.
D-Day is the name for 6 June 1944, when Allied troops landed on the coast of German-occupied France during World War II. It was the largest seaborne invasion in history, with over 150,000 American, British, and Canadian troops landing in Normandy, including 23,000 airborne paratroopers, and involving almost 7,000 ships, boats, and landing craft. But it turns out that the term itself is older, dating to another war, and it is also something of a redundancy.
The term comes out of the American military and is used in the planning for any operation, the Normandy invasion being only the most famous example. The D simply stands for day, so literally, D-Day is day-day. The term is used in coordinating the timing of a military operation. D+0 ("D plus zero"), or D-Day, is the start of the operation. D+1 ("D plus one") is the next day, D+2 is the day after that, etc. Similarly, D-1 ("D minus one") is the day before the operation and used to designate the timing of preparations. H and H-Hour are used in a similar fashion. The advantage of this system is that if the date of the operation is advanced or delayed, the plans don’t need to be revised. In fact, the Normandy landings were originally scheduled for 5 June 1944, but bad weather delayed the operation by one day.
D-Day and H-Hour were not invented in World War II; instead they date to World War I. The earliest citation in the Oxford English Dictionary is from the First Army, American Expeditionary Force in Field Order No. 8, issued on 7 September 1918:
The First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of St. Mihiel salient.
Source:
Oxford English Dictionary, second edition, 1989, s.v. D-Day, n.; H-Hour, n.