Why was it called D-Day?

D-Day has become part of a series of "-Days" in military planning. Some I remember are M-Day, the day mobilization starts. D-Day/H-Hour, the day and time the operation starts. C-day, the day deployment of forces starts. All the -Days are relative and are only assigned real dates when the operation starts. In relative terms, the -Days are valued at "00" and then other values are computed in minus days, such as D-10 (10 days before D00) or plus days, such as C031 (31 days after C00). The automated planning systems are designed to work with the -Days and to accept and interpret assignment of real dates to the 00 day value for each -Day. For example, C00 can be set to 1 Sep 2018 (probably 20180901). Military planner often speak in "C-Day" or "D-Day" language as part of their particular military jargon. For example, "The 345th Engineering Squadron mobilizes on M plus 45 and deploys on C plus 186."

CW5 Jack Cardwell thanks for the read/share of the most interesting.

Somewhere in my past, I understood it to stand for debarkation day.