What they knew
The landings needed a late-rising moon for the airborne drops, a low tide at dawn to expose beach obstacles, and calm enough seas for landing craft. Only a few days a month qualified. The invasion was set for 5 June, but a storm forced a 24-hour postponement.
Group Captain James Stagg, the chief meteorologist, then forecast a short break in the weather for 6 June.
The choice
If Eisenhower waited, the next suitable tides would not come for about two weeks, with thousands of troops already sealed aboard ships and the risk of the secret leaking. If he went and the forecast was wrong, the invasion could fail on the beaches.
In the early hours of 5 June he listened to the final forecast and gave the order.
The consequences
The break came. German forecasters, lacking the same weather data from the Atlantic, did not expect it, and some senior commanders were away from their posts. The later window in June saw one of the worst Channel storms in decades.