D-Day may also refer to Decimal Day in the United Kingdom. , the most famous D-Day.]] Meaning and context 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". By far, the best known D-Day is June 6, 1944 ? the day on which the Battle 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 Operation Ov