In French "Jour J" is mainly used to refer to June 6th 1944, ....
Merci.
I don't know how US forces started using D Day. For all I know we borrowed it from the French or the Brits.
ETA: According to Wikipedia the US Army’s center for military history determined that the first use of the DDay and H Hour terms by the US Army was for the attack on Saint Mihiel salient on 12 September 1917. This was a combined attack by US Army and French troops under the command of General John J. Pershing. I suspect that since the French had already been at war with the Germans for about three years at this point that General Pershing borrowed it from the French.
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