Today is the 72nd anniversary of the U.S. and its World War II allies landing on Normandy beaches in 1944. It was a day of great sacrifice and patriotism by our soldiers and the allies.
The invasion - code named "Operation Overlord" at the time - is better known as "D-Day". Many think the D in D-Day refers to "Decision" or "Deliverance" or even "Designated" - but the truth is actually a bit more mundane.
Over the years, the use of the term "D-Day" has been applied to many different combat operations. In reality, the D exists as a stand-in or placeholder variable for the actual date of the operation. This is valuable in planning for a campaign where the actual date is not finalized. For instance, you could state in the plan that on date "D-1" or "D+7" such and such a thing would happen.
That's the real story of the D in D-Day.