D-Day refers to June 6, 1944, the day Allied forces launched a massive invasion of Nazi-occupied France during World War II, known as Operation Overlord. This pivotal operation marked the beginning of the liberation of Western Europe from Nazi control and was characterized by the landing of troops on the beaches of Normandy, facing fierce German resistance. D-Day set in motion a series of events that would lead to the eventual defeat of Nazi Germany and the end of the war in Europe.