D-Day refers to the date, June 6, 1944, when Allied forces began the invasion of Normandy during World War II. The invasion was a large-scale military operation that involved the simultaneous landings of Allied troops on the beaches of Normandy, France, with the goal of liberating Western Europe from Nazi occupation. The date "D-Day" is used in military terminology to refer to the day on which a planned military operation is to commence, and in the case of the Normandy landings, it was specifically chosen as the day when full-scale combat operations would begin.