D-Day is a military term that refers to the basic date and time from which an important event, usually an attack, could be scheduled. In other words, the “D” in D-Day merely stands for Day. For military planners, the days before and after a D-Day were indicated using plus and minus signs: D - 4 meant four days before a D-Day, while D + 7 meant seven days after a D-Day. The best known D-Day was June 6, 1944, during World War II, when Allied forces landed in Normandy, France.