The Normandy landings (June 6, 1944) marked the beginning of the liberation of Western Europe occupied by Nazi Germany during World War II.
Hitler was always clear that there would be an Allied landing on the French Atlantic coast, specifically in the area of the English Channel. Just as they did with the body found in Huelva, which cost him Sicily, Allied intelligence managed to trick Hitler into believing that the Normandy landing was a diversionary maneuver and that the real one would take place in Calais (almost 400 km further to the North).
Hitler entrusted the defense of the French coast to Erwin Rommel , Desert Fox . Rommel ordered the planting of mines, barbed wire and obstacles, as a defense devil's garden (the garden of hell) in El Alamein (Egypt), and made a study of the tides. At high tide the defenses were covered and their effectiveness was null. So, he devised obstacles that could damage the hull of landing craft even when submerged. 
Defenses
For the Americans it would have been better, as Rommel thought, to attack at high tide to have less beach to cross under enemy fire, but Rommel's traps could destroy the boats and be stranded on the beach, preventing the landing of the rest of the troops. . Therefore, the best situation was one in which the tide was low enough not to cover the booby traps for the demolition teams to locate and open a corridor for landing, but high enough for the boats to unload the troops. and then get out without danger of being stranded by low tide.
The exact knowledge of the tides was too important a matter to be left to chance. This is where our protagonist, the British mathematician Arthur Thomas Doodson, intervenes. . The Allies consulted experts, including Doodson, to find out the best dates for the landing. Doodson had built a machine for predicting the tides , which continued to be used until the 1960s with the advent of computers. With Doodson's machine, it was calculated that the ideal dates for the landing were June 5 to 7.
Doodson's machine