D-Day is code for the day of the Battle, H-hour, for the time. Normandy, was where the Allied Armies landed in France, to begin to free occupied Europe and win the War against Hitler and Germany.
The day picked, was due to the weather and the tides, if the Allies hadn't invaded then, the Invasion, would have to have been delayed, for months, or even a year.
The weather was bad, with many storms at that time of year, but the British Weather experts, found a small opening, that would be perfect for the Invasions.
The German's hadn't seen that, so they weren't prepared for the invasion, and also due to the Allies Plan, to fake the Germans into believing the landings were to be at the Port De Calais.in France, which was the shortest distance between France and England.