My late father, a career military man with the US Army, partook in Operation Overlord, D-Day, as a paratrooper with the 82nd Airborne, and as a participant of the initial "night drops" early in the morning of 6 June 1944. Until his dying day, he said he never believed he would actually make it safely to the ground, for he and his fellow paratroopers had been advised to expect heavy retaliation if their parachutes were detected by German forces located in and/or near their landing areas. My father always said that 6 June 1944 was indeed "The Longest Day" for him, even though he had participated already in the invasion of Sicily and then continued on until the war's end with Overlord's steady move eastward and across the Rhine.
Interestingly enough, my father always said that if the Battle of Kursk (the largest land battle in recorded history, which took place during the summer of 1943 on the Russian Front) had not been won by the Soviet Union, he doubted if Overlord would have been as successful as it was, for that massive Russian victory effectively decimated the majority of the German's tank and aircraft strength, thus leaving the Reich's western front lacking as strong a defense as they had previously planned for the possibility of an Allied invasion. History has shown now that my father was absolutely correct in his assertion.