[D66] D-Day

[A.OUT]

What is the D in D Day stand for?

The D simply stands for "day." The designation was traditionally used
for the date of any important military operation or invasion, according
to the National World War II Museum. Thus, the day before June 6, 1944,
was known as D-1 and the days after were D+1, D+2, D+ and so on.5 jun. 2014

https://www.latimes.com/nation/nationnow/la-na-why-its-called-d-day-story.html