D-Day is a general military term for the day on which an operation or exercise is planned to commence.  The choice of the letter D has no significance, and any other letter could equally be used.  Its only purpose is to provide a point of reference from which all other dates can be reckoned.  D+1, for example, would be the day after D-Day, or the second day of the operation.  This allows all aspects of the plan to be worked out in advance, even though the actual date of D-Day might remain to be decided.

The most famous D-Day was 6 June 1944, the beginning of the Allied invasion of France, although there were numerous others during the Second World War.

Similarly, H-Hour is a general military term for the exact time at which an operation or exercise is planned to commence.  As with D-Day, the actual choice of letter has no significance.  Other designations, such as Zero-hour, can also be used.

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Source:
Jane's Dictionary of Military Terms, compiled by P.H.C. Hayward (pub. Macdonald & Jane's, London, 1975)   ISBN 0-356-08261-X