Been meaning to post this for a while, saw it on FamilySearch – Why Is It Called D-Day? – FamilySearch November 21, 2019: https://www.familysearch.org/blog/en/why-is-it-called-d-day/.
On June 6, 1944, Allied forces invaded German-occupied beaches of Normandy, France. This significant day in history became commonly known as “D-Day,” though the actual meaning of the nickname isn’t as well understood.
There are a few conjectures as to what the “D” stands for. Some have said it is an abbreviation for “departure” or “decision.” Others claim that it stands for “doomsday.” However, according to the United States military, the origin of the popular nickname has more to do with their terminology.
The Meaning of “D-Day”
The phrase “D-Day” was used by the Army to designate a specific starting date for field operations. Other phrases, like “D+2” would have referred to two days after the initial “D-Day.” This designation suggests that the letter “D” doesn’t stand for anything in particular outside of “Day” and that it served only as a point of reference.