"D-Day" and "H-Hour" are general terms used for the day and hour to mark the beginning of an important event.
By far the most well-known D-Day is June 6, 1944, when the Allied invasion of German-occupied France began in WWII.
The "D" was used to mark the day that a particular operation was to begin. Each operation had a D-day and an H-hour.
Because D-Day of Operation Overlord was the largest amphibious assault in military history, it became the popular expression to refer to June 6, 1944, and was not used to mark the first day of an operation thereafter - as far as I know. It basically took on the persona that the phrase "9/11" has taken to refer to the terrorist attacks on the World Trade Center and Pentagon.
General Eisenhwer and Winston Churchill may have tried to give the "D" an actual meaning for the benefit of the press and the civilians, but previous to that, it did not stand for anything except "day" (as noted previously).
Indeed, D = Day and H = Hour, but I will elaborate a little: The invasion (or any major operation) was planned way in advance without a date being assigned for commencement. This was done for security reasons and to keep the element of surprise so that the enemy response would be minimal. For purposes of planning, you assumed the operation would start at D-Day and H-hour, with the day and time to be determined later. Then you can plan for how things will proceed, starting from Day 1 - 0 hour and start counting as in D+1 day, D+2 day, etc. Only at the last minute did anyone know what day the whole thing would actually take place.
With June 6, 1944, for example, they had to wait on the weather, amongst other things.
dday means doomsday
According to the Department of Defense Dictionary of Military and Associated Terms, D-Day means: "D-day. The unnamed day on which a particular operation commences or is to commence." So the D means nothing.
For Skirmish Dday you have to call. I assume it is the same for Oklahoma and other big dday games also.
Was d-day a success? Yes. DDay is this
how many tanks were there durind dday
After the D-Day, the days of the German resistance were numbered.
it stands for death day because so many died
The outcome of D-Day was the Allies won the battle and took the beach.
About 3000 people died on Dday.
D-Day is a generic term, but provided this is in regards to the Allied invasion of Europe, the D-Day landing began on June 6, 1944.