what does the d in d day stand for

The "D" in D-Day simply stands for "day". The term was used to describe the first day of any large military operation, and the designation was used for the day of any important invasion or military operation. For military planners (and later historians), 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.