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.