D-Day was originally set for June 5 but had to be postponed for 24 hours due to bad weather.
The planning team responsible for the invasion of Normandy had to consider the weather, the moon, and tides when assigning a date for D-Day. Air operations required clear skies and a full moon for good visibility. Naval operations required low winds and calm seas to safely transport troops ashore. Ground troops needed to land at low tide when German beach obstacles were exposed and easier to deal with.
D-Day required the best combination of these factors. Military planners relied on information from meteorologists and other specialists, who advised that D-Day should fall somewhere between 5 and 7 June 1944. D-Day was set for 5 June, but Supreme Commander General Dwight D Eisenhower knew that the weather could be critical in determining whether the invasion went off as planned.
The decision to postpone was a difficult one, as any delay made it increasingly difficult to keep the operation a secret. If the weather did not improve, D-Day would have to be delayed until the tides were again in the Allies' favor. This would not happen for another two weeks. But over the course of 4-5 June, Stagg predicted a temporary break in the weather. Based on this information, Eisenhower ordered that the invasion proceeds on 6 June.