D-Day is a standard military term of art for the day on which an operation will start, along with the more precise H-hour and M-minute. While it is commonly associated with the Battle of Normandy, it is not specific to that operation; every Allied invasion had a D-Day.
Since the exact timing is often one of the most sensitive part of an operations, the operation order may say:
- Direct support artillery will be in place at D-1
- Beach reconnaissance parties will land at H-3 hours and egress at H-2:45 hours
- Bombardment will begin at H-1 hour on D-Day
- Landing craft for the first wave will marshal at H-2 hours
- The first wave will land at H hour
- The second wave will land at H+25 minutes...
Actual values for D and H will be transmitted separately, on a need-to-know basis. In practice, as with the Normandy invasion, dates are relative; the specific start was rescheduled due to weather and other factors, but the orders did not need to be reprinted.
Commonwealth countries sometimes use X-Day rather than D-Day.