D-Day refers to two separate events that are related to each other. The first use of the word is to refer to any day in which an initiative is taken by the military. Most commonly, this alludes to combat attacks, but it can also signal the beginning of peaceful operations, invasion procedures, and takeover maneuvers. The term was first used during World War I on a written order to attack St. Mihiel Salient on 7 September 1918.
Despite different interpretations give to the first "D" in the name, the truth is that it does not stand for anything, even among military personnel. D-Day is one of several military denominations using letters of the alphabet. C-Day (or Candy Day) is the day deployment orders are given; E-Day refers to the beginning of any NATO exercise deployment; H-Hour is the exact time an operation starts; and V-Day marks Victory Day, or the day a military attack is finished.
D-Day is also the most common name given to 6 June 1944, the day the Battle of Normandy began. This was the day the Allies arrived in Europe to help liberate the continent from Nazi occupation. The full name for the battle is "1944 D-Day Operation Overlord" as a reference to the largest sea invasion in military history. On this day, about 156,000 troops arrived in Normandy, France, from England. After parachute landings and air and naval attacks, troops arrived via water and took over Normandy, starting a fight that would last for over two months and ended with the liberation of Paris at the end of August.
The term has such an emotional connotation for World War II veterans and survivors that the military has avoided using the term officially since 1944. While it may be used among military personnel, a different denomination is often used in open speeches and public announcements. For example, L-Day was used to refer to the invasion of Okinawa (March 1945), and A-Day to refer to the invasion of Leyte (October 1944).