Did you know...
... that the "D" in D-day stands for "D"ay?
When generals and admirals plan military operations, the don't know when or even if those operations will take place, so they arbitrarily assign the letter "D" to an arbitrary Day the operation will commence and the letter "H" to the hour the operation will commence. They can then plan the timing of their operation by arbitrarily assigning dates like D+1 or D+2 to the first and second day after the operation commenced. Similarily, they assign the nomenclature H+1 or H+2 to the first and second hours after an operation commenced for shorter operations.
So, the famous "D-day" of June 6, 1944, when Allied forces invaded the coast of France by an armada of boats from England is named after the military system of arbitrarily assigning the letter "D" to the date a military operation commences. In this case, "D" turned out to be June 6, 1944.
It's often said that Eisenhower himself often refered to D-day as the date that Allied forces "disembarked" to the coast of France, and the word "disembark" means to go ashore from a ship. So, history buffs will insist that the D in D-day means "disembark". However, it's commonly assumed that Eisenhower was fully aware of the use of the letter "D" amongst military planners to represent the day an operation starts, and was not intending to redefine the meaning of that letter in his speeches. He simply used the word "disembark" because it meant to go ashore from a ship. The fact that the word "disembark" starts with the letter D is merely a coincidence.