D-Day / H-Hour / J-Day
D-Day is probably best known as a 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 has an older, more general meaning, 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. And before D-Day, the term J-Day was used to signify the beginning of an operation. The J is from the French jour (day), and the American military practice of planning the timing of an operation in this fashion was adopted from the French, with whose army the WWI American Expeditionary Forces often had to coordinate operations. The use of the letter J to signify the day of an attack appears in a French operations order from October 1917:
Les coups de mains sont de plus en plus nécessaires à mesure que le jour J—le jour de l'attaque—se rapproche.
(The sudden attacks in force become more and more necessary as the day J—the day of the attack—approaches.)
The earliest American use of J-Day and H-Hour that I have found is in an operations plan of the U.S. 16th Infantry Regiment, part of the 1st Division, from 21 February 1918:
Artillery: From J minus 5 day to J day—Cutting gaps in the enemy’s wire at points other than that of the real objective, namely, Salient LAHAYVILLE and Salient 2520.
[…]
Box Barrage—A box barrage of 45 minutes duration will be held at H hour as shown on the sketch. At H plus 25 the box barrage will close down to the enemy’s first four lines which will be heavily shelled till close of operation.
D-Day enters U.S. Army nomenclature later that year. 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.
An alternative origin for J-Day may be that it stands for jump-off day. The phrase jump off was used by the AEF in World War I, but it is more likely that the French practice was adopted by the less-experienced American forces and then later anglicized to D-Day. The use of J-Day in English was limited to official operations plans and orders—or at least I’ve found no examples of it in less-official sources. If it were from jump-off day, then one would expect to see it in wider use.
Sources:
Headquarters, 16th Infantry. “Plan for Raid on Richecourt Salient.” 21 February 1918. World War Records First Division A.E.F. Regular, vol. 10. First Division Museum.
Oxford English Dictionary, second edition, 1989, s.v. D-Day, n., H-Hour, n.
D'un poste e commendement (P.C. du 21e C.A.) Bataille de l’Ailette (23 octobre–2 novembre 1917). Paris: E. Flammarion, 1918, 61. HathiTrust Digital Archive.
Reitan, Peter. “‘H-Hour’ and ‘J-Day’, 21 Feb 1918.” ADS-L, 4 May 2025.
Photo credit: Herman V. Wall, 1944. Wikimedia Commons. Licensed under a Creative Commons Attribution-Share Alike 4.0 International license.
