D-Day is a term used in military planning 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, etc. This allows scheduling a sequence of events before the start date is chosen. The days after a D-Day are D+1, D+2, D+3, and so on. Several different events in military history were called D-Day such as the Invasion of Normandy (Operation Overlord).
The most famous D-Day was on the morning of June 6, 1944, when the largest naval attack in military history took place. It was in France during World War II, when Allied forces crossed over the English Channel from England to the Normandy Coastline. This attack was code named Operation Overlord, led by American General Dwight D. Eisenhower, in reply to the failed Dieppe Raid (British Attack), in 1942. The Allies (Britain, Canada, and America in this case), breached the Normandy coastline, via the German outpost, bunker, and fortification chain, the Atlantic Wall, which was a turning point in World War II, though over 3,500 men were lost. Less than a year later, the Allies entered Nazi Germany, and Germany surrendered to the Allies.
The invasion started with airborne landings at 3:30 AM, and the seaborne invasion started at 6:30 AM. The invasion was planned for months, and was scheduled for June 5, but was delayed by one day due to bad weather. Once the invasion succeeded,