The phase of the moon (full) and the level of the tide (three hours past its ebb) would dictate the invasion’s start.
For a few days each month those conditions were possible. A full moon would provide sufficient light. The tide had to be low enough to allow those manning the landing crafts to see German-placed obstacles on the French shore but high enough for troops to avoid too much unprotected beach. The best day in June, 1944, was the 5th - a Monday.
But would the weather cooperate?