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The terms D-Day and H-Hour are used for the day and hour on which a combat attack or operation is to be initiated. They designate day and hour for an operation when the actual day and hour have not yet been determined or announced. The letters are derived from the words for which they stand, "D" for the day of the invasion and "H" for the hour the operation actually begins.
When used in combination with figures and plus or minus signs, these terms indicate the length of time preceding or following a specific action. Thus, H-3 means 3 hours before H-hour, and D+3 means 3 days after D-day. H+75 minutes means H-hour plus 1 hour and 15 minutes.
Planning papers for large-scale operations are made up in detail long before, specific dates are set. Phased orders are planned for execution on D-Day or H-Hour minus or plus a certain number of days, hours, or minutes.
According to the U.S. Army's Center of Military History, the earliest known use of these terms is in Field Order Number 9, First Army, American Expeditionary Forces. It is dated September 7, 1918: "The first Army will attack at H hour on D day with the object of forcing the evacuation of the St. Mihiel Salient."
D-Day for the invasion of Normandy was set for June 5, 1944, but it actually occurred on June 6. Therefore, D-Day, as it applies to Overlord, is June 6, 1944.
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