D Day
For a description of the events of 6th June 1944, see Normandy Landings.
D-Day may also refer to Decimal Day in the United Kingdom.
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".
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.
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.
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
Official U.S.
Twelfth Army situation map for 2400 hours, 6 June 1944.
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.