My second question is, "When did it occur?". As stated in my last answer, it occured on June 6, 1944.
My last question was, "What does it stand for?". D-Day stands for, simply, a day that a major operation was to begin, in military terms.
There is also a video on http://www.army.mil/d-day/. If you have time to watch it its really good. Its about the army personal that took part in d-day! One great video and the stories!!!!!