The term “D-Day” is typically used to refer to the day, June 6, 1944, on which the Allied invasion of France began in World War II.
Early in WWII, Germany had conquered essentially all of Western Europe. Since the Allies’ war goal was to force the unconditional surrender of the Germans, they had to invade Europe so as to push Germany out of the countries it had conquered. The D-Day landings were the beginning of the main invasion of Europe.
The D-Day landings took place on the coast of Normandy. Allied forces (American, British, and Canadian) landed along a 50-mile stretch of coast. On this day, about 160,000 soldiers landed, making this the biggest amphibious invasion ever. Some of this number were brought by air, either as paratroops or in gliders that landed in fields.
The D-Day landings were a success for a number of reasons. By this time, the Allies had air and sea superiority and were therefore able to offer excellent air and naval support for the landings. They had also deceived the Germans into believing that the landings would occur in a different part of France. Finally, the weather was bad enough that there was some question as to whether the invasion could happen, also helping to ensure that the Germans were unprepared.