Short answers, pulled from the story.
Operation Overlord began on the 6th of June 1944 with American General Dwight D. Eisenhower appointed as commander of Supreme Headquarters Allied Expeditionary Force while British General Bernard Montgomery took command of land forces.
Allies selected Normandy because it permitted simultaneous threats against Cherbourg western ports and an overland push toward Paris whereas Pas-de-Calais remained heavily fortified by German forces with few opportunities for expansion due to surrounding rivers and canals.
Engineers designed two Mulberry harbours consisting of floating outer breakwaters inner concrete caissons known as Phoenix breakwaters and several floating piers supplemented by blockship shelters codenamed Gooseberries to overcome the absence of natural deep-water ports on the Normandy coast.
Nearly 160,000 troops crossed the English Channel on the 6th of June 1944 as part of Operation Overlord which stands as the largest seaborne invasion in history.
The U.S. First Army launched Operation Cobra on the 25th of July advancing further south to Avranches by the 1st of August after securing territory in the Cotentin Peninsula south as far as Saint-Lô.