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D-Day is a term used in the military to mean the actual day a major operation or event is to begin. The days leading up to a D-Day are called D-1, D-2, D-3, and so on. The days after a D-Day are D+1, D+2, D+3, and so on.
There were many days in military history that were called D-Day. The most famous D-Day was June 6, 1944 when the biggest sea to land attack in military history took place in Normandy during World War II when the allied forces crossed over the sea from England and landed on the beach. This attack was named Operation Overlord, led by American General Dwight Eisenhower. The Allies (Britain, Canada, and America) attacked the Nazis guarding the beach, because France, at the time was occupied by Germany. The allied forces won the battle, which was important in the long term of ending World War II, though over 3,500 men were lost while attacking the beaches. Germany would eventually surrender, and the Allies, having won the second World War, would go on to free France from the Germans.