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 but the most famous one was the Invasion of Normandy, called Operation Overlord. This 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. Operation Overlord was led by American General Dwight D. Eisenhower. 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.
The invasion started with airborne landings at 3:30 AM, and the seaborne invasion started at 6:30 AM and took 19 hours. Operation Overlord was planned for months, and was scheduled for June 5, but was delayed by one day due to bad weather. Once the invasion was over the Allies were bound for Paris in a couple of months, fighting through France and taking key cities such as Caen. Less than a year later, the Allies entered Nazi Germany, and Germany surrendered to the Allies.