D-DAY

Battle plans for the Normandy Invasion, the most famous D-Day.

D-Day is a term often used in military parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours for similar terms. The initial D in D-Day has had various meanings in the past, while more recently it has obtained the connotation of "Day" itself, thereby creating the phrase "Day-Day", or "Day of Days".[1]
By far, the best known D-Day is June 6, 1944 — the day on which the Invasion of Normandy began — commencing the Western Allied effort to liberate mainland Europe from Nazi occupation during World War II. However, many other invasions and operations had a designated D-Day, both before and after that operation.[2]
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 the day and hour of the operation when the day and hour have not yet been determined, or where secrecy is essential. There is but one D-Day and one H-Hour for all units participating in a given operation.
When used in combination with figures, and plus or minus signs, these terms indicate the point 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. Thus, orders are issued for the various steps to be carried out on the D-Day or H-Hour minus or plus a certain number of days, hours, or minutes. At the appropriate time, a subsequent order is issued that states the actual day and times.

History
The earliest use of these terms by the U.S. Army that the Center of Military History has been able to find was during World War I. 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."
D-Day for the invasion of Normandy by the Allies was originally set for June 5, 1944, but bad weather and heavy seas caused Gen. Dwight D Eisenhower to delay until June 6 and that date has been popularly referred to ever since by the short title "D-Day". (In French, it is called Le Jour J or, occasionally, Le Choc.) Because of this, planners of later military operations sometimes avoided the term. For example, Douglas MacArthur's invasion of Leyte began on "A-Day", and the invasion of Okinawa began on "L-Day". The Allies proposed invasions of Japan that would have begun on "X-Day" (Kyūshū, scheduled for November 1945) and "Y-Day" (Honshū, scheduled for March 1946).

Notes
^ "D-Day". The Oxford Companion to World War II. (2005). Oxford: Oxford University Press. 220. ISBN 0-19-280666-1. Retrieved on 2007-09-07.
^ Hakim, Joy (1995). A History of Us: War, Peace and all that Jazz. New York: Oxford University Press. ISBN 0-19-509514-6.
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