There were many months of planning before the June 6. 1944 invasion. Everything from the fictitious First Army Group (FUSAG) with General McNair and General Patton to
inflatable tanks. Even fake radio traffic was used to convince the Germans that the invasion would occur in Northern France (Operation Bodyguard) or in Norway (Operation Fortitude).
Operation Neptune also known as D-Day was the Normandy Invasion. While the British and Canadian beach heads of Sword, Juno and Gold had some action it was nothing compared to Omaha Beach. The Americans faced a very well trained German infantry. Omaha Beach as well as Pointe du Hoc were in Allied hands after several days of fighting.
The weather played a huge part in the invasion plans. There was a storm on June 5th that delayed the invasion plans. The next day June 6 is when the invasion started. If they had waited until later in the month they would have faced a much larger storm.
For those who want to read more please go to: en.wikipedia.org. There is also shaef.org