Today marks the 71st anniversary of the Allied invasion of Normandy, France, designated at the time as Operation Neptune but known in popular culture today as D-Day. Why is that, and what does the D stand for?

Lots of people think it means “Decision Day,” “Doomsday” or even “Death Day.” And considering and during the Operation Overlord campaign that followed, those things all make sense.

Despite a lot of disagreement among military historians, according to , the D probably just stands for Day. No, not “Day Day,” but rather a sort of measurement of time:

This coded designation was used for the day of any important invasion or military operation. For military planners (and later historians), the days before and after a D-Day were indicated using plus and minus signs: D-4 meant four days before a D-Day, while D+7 meant seven days after a D-Day.

So it’s not the “D” that matters in D-Day, but rather the “Day,” which designates the actual date of the event itself.

But as I mentioned, not everyone agrees with this. Even the French and one of Eisenhower’s own generals had different explanations:

Many explanations have been given for the meaning of D-Day, June 6, 1944, the day the Allies invaded Normandy from England during World War II. The Army has said that it is “simply an alliteration, as in H-Hour.” Others say the first D in the word also stands for “day,” the term a code designation. The French maintain the D means “disembarkation,” still others say “debarkation,” and the more poetic insist D-Day is short for “day of decision.” When someone wrote to General Eisenhower in 1964 asking for an explanation, his executive assistant Brigadier General Robert Schultz answered: “General Eisenhower asked me to respond to your letter. Be advised that any amphibious operation has a ‘departed date’; therefore the shortened term ‘D-Day’ is used.”

Luckily, VE-Day and VJ-Day are a heck of a lot easier to figure out.

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