On D-Day, 6 June 1944, Allied forces launched a combined naval, air and land assault on Nazi-occupied France. The 'D' in D-Day stands simply for 'day' and the term was used to describe the first day of any large military operation.
Most argue it was merely a redundancy that also meant day, but others have proposed everything from departure to decision to doomsday. According to the U.S. military, D-Day was an Army designation used to indicate the start date for specific field operations.