In early 1944, Allied commanders were focusing on the month of June for the invasion of Europe. Based on the phases of the moon and the expected tides, two windows of opportunity were identified: June 5-7 and June 17-18. The choice of when to go was very much left to weather forecasters.

The first few days of June 1944 were very stormy along coastal France. There was no chance to go on June 5, the original date Eisenhower had chosen. But forecasters predicted a short break in the weather for June 6, so Eisenhower gave the go-ahead for that day.

As it turned out, the weather was marginal - choppy seas and gray skies - but nature cooperated enough so that the Germans - who expected an invasion only in fair weather - were taken by surprise. In retrospect, had the Allies not chosen June 6, the invasion would have likely faced a long delay because one of the worst June storms in English Channel history hit during the other window of opportunity later in June, and lasted for five days. Looking back, Eisenhower wrote: "I thank the gods of war we went when we did."