Short answers, pulled from the story.
General Dwight D. Eisenhower made the decision to launch the invasion on the 6th of June 1944 after a single hour of consultation with his meteorological team. Group Captain James Stagg of the Royal Air Force predicted that the weather would improve enough for the invasion to proceed on that date.
Operation Bodyguard was a massive intelligence operation designed to systematically mislead the German high command about the location of the main attack. The Allies created a fictitious First United States Army Group under Lieutenant General George S. Patton to deceive the Germans into believing the main attack would take place at Calais.
The Luftwaffe could muster only 815 aircraft over Normandy in comparison to the Allies' 9,543 aircraft. This disparity left Field Marshal Erwin Rommel unable to expect effective air support for the German defenses.
The five beachheads were not connected until the 12th of June 1944. By that time, the Allies held a front around 100 kilometers long and 10 to 15 kilometers deep.
Civilian casualties on D-Day and D+1 are estimated at 3,000. The Germans had ordered French civilians other than those deemed essential to the war effort to leave potential combat zones in Normandy.