What does the D in D-day stands?

While D-Day is used to refer to the Normandy landings, it is simply a reference of time for an operation. An operation begins on a "D-Day". D+1 is one day after the begining, D+2 is two days, and so on.

H-Hour can also be used: H+1 is one hour after an operation, H+2 is two hours, and so on. They aren't usually used together. 1 1/2 days would be H+36, not D+1, H+12.