Nowadays, the word is used to mean the day chosen for the commencement of an important activity.
*The players are preparing themselves for D-day tomorrow. *Tomorrow is D-day. We are launching our new product. So Friday is D-day, is it?
The “D” in D-day stands for “day”. So when you say “D-day” what you are actually saying is “Day-day”. The expression was first used during World War I, and not as many people think in the Second World War. The famous D-day was of course on June 6, 1944.