(n.) 6 June 1944, the day on which Allied forces invaded northern France during the Second World War
Etymologically, there’s a longstanding myth that the D of D-Day stands for something along the lines of ‘disembarkation’, ‘decision’, ‘deployment’, or even ‘Deutschland’ or ‘Doomsday’. But in fact, it doesn’t stand for anything.
While military operations are being planned, it’s not always clear from the outset when they’ll actually take place. As a result, their future start date—whenever that may be—is simply referred to as D-Day, and this title acts as a placeholder until a specific date can be finalized.
If anything, the D of D-Day could be said to derive from the word ‘day’ (indeed the French equivalent is J-Jour, and the exact time an operation takes place is known as H-Hour) but it certainly can’t be said to stand for it.
Not only that, but the term D-Day is also a lot older than most people think. The earliest record we have of its use dates not from the Second World War, but from the First, and an American military order sent out 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.
Saint-Mihiel is a small town in the Meuse department of northeast France, that for three days in September 1918 was the site of one of the most important United States military operations of the entire First World War. Under the command of US Army General John Pershing, an enormous body of American Expeditionary troops—including thousands from the newly-formed United States Army Air Service, now the US Air Force—secured a decisive Allied victory over an ill-prepared and chaotic German contingent.
The Battle of Saint-Mihiel lasted from 12-15 September, during which more than half a million US soldiers, alongside 110,000 French troops, fought to secure the strategically significant Saint-Mihiel salient (a technical term for a narrow, isolated strip of land projecting from one region into another) in the hope of eventually recapturing the larger French city of Metz. As it happens, the attack on Metz was never realised, and as the German forces continued to crumble the War came to an end just weeks later, on 11 November 1918.
The term D-Day continued to be used intermittently throughout the 1920s and 30s, until it became all but permanently attached to Operation Neptune—the military codename of the decisive Normandy Landings—on 6 June 1944.