Montgomery realised that both flanks of the sea borne landing had to be covered, he decided that airborne troops would be dropped at night to secure these positions. The British 6th. Airborne would be dropped to the eastern flank, and the American 101st. and 82nd. Airborne would be dropped to the west.
The beach landings were to take place at dawn, therefore, the airborne landings had to take place at night. A full moon was required for this to be successful.
The only time the moon, tides, and winds would be in the right phase was for three days, starting Monday June 5th.

Erwin Rommel, Monty's old adversary was given command of Army Group B in July 1943, taking over the defence of Belgium and northern France in December that year. Rommel was of the opinion that the likely invasion site would be at the Pas de Calais and he personally supervised the construction of the  Atlantic Wall.
He knew that whenever and wherever the invasion force landed, it had to be defeated on the landing beaches, that was imperative. He had obstacles constructed on the beaches the full length of the Normandy coast, these were designed to rip the bottom out of any landing craft, and many did.

The obstacles that were positioned on the landing beaches, were designed so that landing craft approaching at high tide would not see them. Some of them had mines attached, the result of hitting these was catastrophic and many men died never reaching the beach.

Rommel realised that an invasion of the beaches would also be accompanied with a massive airborne assault,he had areas of land flooded to hinder the progress of airborne troops, this was very successful for him, as many of the troops perished in these swamp areas, laden down with heavy equipment and ignorant of the trap.
ROMMEL KNEW THE FIRST TWENTY FOUR HOURS WERE VITAL FOR BOTH SIDES AS HE TOLD HIS MEN
                         "IT WILL BE THE LONGEST DAY"

On his appointment as Supreme Allied Commander, Eisenhower set up his new power base, known as SHAEF short for "Supreme Headquarters Allied Expeditionary Forces" It was their responsibility to plan and oversee the successful invasion of Europe. SHAEF's ground forces were the 21st. Army Group. The title being derived from the joining of it's two components, ie. The British 2nd. Army and the U.S. 1st. Army.
As an American was Supreme Commander, Winston Churchill had the right to delegate a British officer to take

SHAEF had set the invasion date for Monday 5th. June, the troops who would lead the invasion were moved to there embarkation camps in readiness. On June 3rd. the first naval units began their deployment. On the 4th. June the English Channel had its worst gales in twenty years!! The invasion had to be postponed for 24 hrs. Many of the ships and landing craft had actually set out to the rendezvous points and had to hold station during this horrendous weather, needless to say the men aboard suffered appalling conditions, not the ideal preparation for battle.
At Shaef headquarters June 4th. the meteorological assessment was that the weather was starting to clear and would remain so over Normandy on the night of 5th. June and into the morning of the 6th. Eisenhower asked for opinions, there was a mixed opinion.
Eisenhower then said "I am quite positive we must give the order, I don't like it , but there it is,  I don't see how we can do anything else"
                                              D Day was set, 6th. June.                       O.K. Lets Go...