"D" is short for "Day" in military terms, and D-Day signifies a particular date when a major operation was to begin. The days before a D-Day is signified as D-1, D-2, D-3, etc., and the days after a D-Day are D+1, D+2, D+3, etc.
There were many days in military history that were called D-Day. The most famous D-Day was June 6, 1944 when the biggest amphibious (from the sea to the land) attack in military history took place in Normandy, France during World War II. This attack was codenamed Operation Overlord. The Allies (Britain, Canada, and America) attacked Europe occupied by Nazi German. It was very important for the Allied success in World War II, though many men were lost when the beaches were taken over (3,500 to be precise). After that, about two months later, after very hard fighting and many killed, the Allies won. They freed France from the Germans. Less than one year later Germany surrendered. June 6 is very important to many people because of what happened.
Many people think D-Day stands for Decision Day, but it does not, the first letter is just to show the importance of the day.
D-Day had a fake operation called "Operation Fortitude".
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