From the History Channel:

Although the term D-Day is used routinely as military lingo for the day an operation or event will take place, for many it is also synonymous with June 6, 1944, the day the Allied powers crossed the English Channel and landed on the beaches of Normandy, France, beginning the liberation of Western Europe from Nazi control during World War II. Within three months, the northern part of France would be freed and the invasion force would be preparing to enter Germany, where they would meet up with Soviet forces moving in from the east.

With Hitler's armies in control of most of mainland Europe, the Allies knew that a successful invasion of the continent was central to winning the war. Hitler knew this too, and was expecting an assault on northwestern Europe in the spring of 1944. He hoped to repel the Allies from the coast with a strong counterattack that would delay future invasion attempts, giving him time to throw the majority of his forces into defeating the Soviet Union in the east. Once that was accomplished, he believed an all-out victory would soon be his.

On the morning of June 5, 1944, U.S. General Dwight D. Eisenhower, the supreme commander of Allied forces in Europe gave the go-ahead for Operation Overlord, the largest amphibious military operation in history. On his orders, 6,000 landing craft, ships and other vessels carrying 176,000 troops began to leave England for the trip to France. That night, 822 aircraft filled with parachutists headed for drop zones in Normandy. An additional 13,000 aircraft were mobilized to provide air cover and support for the invasion.