where did the term d day come from
D-Day (military term) The earliest use of these terms by the United States Army that the U.S. Army Center of Military History has been able to find was during World War I.[3] In Field Order Number 9, First Army, American Expeditionary Forces, dated 7 September 1918: "The First Army will attack at H hour on D day with the object of forcing the evacuation of the St. Mihiel Salient."