The weather
Timing was everything. The armies wanted a high tide so that the amount of exposed sand would be less. The navies wanted the water level itself to be low, so that submerged mines and other hazards were easier to identify. And the success of the airborne missions would be greatly improved by a late-rising moon, giving paratroopers more darkness with which to work. Allied planners decided that a late-rising full moon would the best of all of those worlds. Based on those considerations, meteorologists advised that the number of serviceable days in a month that would be preferable was six. The timing didn't work for May 1944, and planners looked to the next month. Every month that went by meant more and more death and destruction at the hands of the Axis powers, yet Allied leaders wanted to give themselves the greatest possibilities for success.

The original date planned for the invasion was June 5. Weather forecasts for that day predicted rough seas and overall terrible weather in general, certainly not the kind that lent itself to transporting large numbers of men on ships across open water.

Eisenhower and his commanders decided to postpone the attack for a day. Because of the precise needs with regard to tides, the moon, and other concerns, military planners had advised that only a few days each month were suitable. If the attack did not take place on June 6, then the next clear window was two weeks away. Late in the day on the 5th, a meteorologist predicted a break in the storm pattern of several hours. That was enough for Eisenhower. D-Day was the 6th.

The buildup
Once the Allied forces had gained control of the North Atlantic, they could more easily ferry troops from Canada and the U.S. to the U.K. New World troops numbering in the hundreds of thousands arrived on the shores of England, overwhelming the existing infrastructure. American ships sent over extra railroad engines and cars in order to transport the extensive number of men and materiel from place to place.

To get to the points of departure, the tens of thousands of soldiers who left for the beaches of Normandy travelled on foot and by vehicle through the narrow roads of southeast England for severals before June 6. Some soldiers arrived days ahead of time; others arrived just in time. Equipment, supplies, weapons, and food and drink were plentiful, as was organization.

The departure
Eisenhower said to his troops, "You are about to embark upon the Great Crusade, toward which we have striven these many months. The eyes of the world are upon you."

Ships left a number of harbors, including Bournemouth, Eastbourne, Plymouth, Portsmouth, Southampton, Torquay, and Weymouth. After a stop off at the Isle of Wight, it was all hands on deck for the trip south to Normandy. The landing craft traveled through a relatively narrow stretch of water known as the Spout.

Transporting and supporting those troops were nearly 7,000 seagoing vessels, more than half of which were landing craft; among the other types were battleships, cruisers, destroyers, minesweepers, and submarines. Also supporting the landings were nearly 12,000 aircraft.

Also traveling through the Channel was a very large amount of concrete, in the form of two "Mulberry" artificial harbors and five "Gooseberry" breakwaters. The Normandy beaches had no harbor, so the Allies had chosen to provide their own.

Departure was soon after midnight. The seas were rough, with five-foot waves; a great number of the Allied force got seasick.