The Army and The National World War II Museum have given two explanations. First, it is simply a designation used to indicate the day of an operation as well as the days leading up to it and after it. For example, June 5, 1944, would have been D-1 and June 6, D-Day. Second, ‘D’ stands for ‘departed date’, meaning the date an operation began. So in the case of WWII, there were multiple D-Days for the various amphibious assaults.