D-Day was more about logistics, good luck and a lot of help from the big guy upstairs. If I remember correctly it took 2.5 years for manufacturing (USA and UK) to ramp up the building of the equipment needed. During this time training of volunteers and draftees to become soldiers was carried out. All these men/women and the equipment were transported then crammed into areas in the UK. Allied forces training continued. Before deployment, the civilian population, near the encampments were locked-in and restricted from travel, telephone and mail (communications black-out). Roads, rivers and sea were only for military travel. The big bottleneck was crossing the channel, in secret with the largest armada, then desperately trying to get onto and off the beaches, so that others can come in. Once on land, the flow of men, material, equipment, fuel and food had to continue until Europe was back in the hands of the Allies. The potential was there for a massive disaster but it didn't happen.
Here is a good example. From dark to dark, of June 6th, the number of planes in the air, if averaged, would have been one plane taking off from England every 5 seconds all day/night long. This included return for reloading and refueling then off again. People didn't need to be told that the evasion of Normandy was happening. They could hear and see it.
