63 years ago today, Allied forces began invading France. It was neither the first nor the last time the military has used the term D Day to refer to the day set for launching a specific tactical operation, but the date June 6th, 1944 has become associated with a specific sense of D-Day. (来源：专业英语学习网站 http://www.EnglishCN.com)

We've mentioned before that the D in D-Day stands for "Day;" designating the launch date of an operation with the name D-Day maintains secrecy while allowing serious planning. D-Day does not stand alone; other letters have long been paired with the word day to give us sometimes cryptic, sometimes obvious descriptions.

L-Day (sometimes thought to be short for "Landing Day") referred to the invasion of Okinawa on April 1st, 1945. J-Day, for reasons unknown, was used throughout the second world war to refer to the day an assault (especially an amphibious assault) was scheduled. M-day, short for "mobilization day," has two more general senses: "the day on which military mobilization begins or is postulated to begin;" and "the day on which actual hostility breaks out at the commencement of a war." Finally, of course, there's V-day, where V represents "victory."