What is the etymology and reasoning behind the US Military term,"D-Day"?

Solution 1:

The key to understanding "D-Day" is in how other days in the operation are referenced. The "D" does refer to "day", and in planning a complex operation, certain things have to happen on or by earlier dates; these are scheduled as, say, "D-7" for seven days before the op.

These relative date references are more useful than absolute dates for two reasons: one, you don't have to rewrite the whole plan if the target date changes, and two, you can keep the target date secret while discussing logistics with a lot of subordinates.

So all the other days in the op are "D minus 7" or "D plus 3". The target date is "the day we're calling D", or "D-Day."

Solution 2:

According to the U.S. Army (here) it simply means "Day". "H-Hour" is also used, with the "H" meaning "Hour". I'm not quite sure why the redundancy exists; but that's how it goes, I guess.