D-day stands for Operation D-Day which was the largest military operation in world history where Brittish and American Soldiers landed at Normandy and began the invasion of France which later resulted in the victory of World War II.
Many of the first landing parties were anihilated by machine gun fire. If you seen movies such as saving private ryan, this films opening depicts the landing on D-day, after today,60+ years ago during the war, days in the army were known as "Dday+insert number here" to list how many days since the operation they were in europe