What Does the ‘D’ in ‘D-Day’ Mean?

Once you learn why we call it "D-Day," you won't be surprised by the meaning of "H-Hour."

Joe Muscolino, Writing for
2-minute read
Episode #663
the d in d-day


The 75th anniversary of D-Day is coming up in just a few months and that made me think about the origins of the word D-Day. Why do we call it “D-Day”? What does the “D” stand for? 

The Meaning of D-Day

Let’s start by getting one thing out of the way: “D-Day” doesn’t actually refer to a single date in history. The military term “D-Day” is actually a generic term that can refer to any day on which a particular plan of attack occurs. Over time, the word has become shorthand for the daring 1944 Allied invasion of Normandy during World War II. (And no, the “D” doesn’t stand for “daring.”) The origins of the word date back to at least World War I, when an American field order wrote in 1918: “The First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of the St. Mihiel salient.”

So what’s the “D” stand for? Well, it’s a little anticlimactic. The “D” actually stands for “day.” That’s right. When you say “D-Day” you’re essentially saying “Day Day.” According to the National World War II Museum, the shorthand is used in place of an actual date for the sake of secrecy. Should military intelligence fall into the wrong hands, the enemy will be none the wiser. 

The Days Before and After D-Day

A naming convention was formed for any dates surrounding D-Day. Days that precede or follow D-Day are measured by their distance from D-Day with a plus or minus sign. For instance, if you’re writing to a General about plans on the day that falls before “D-Day,” you’d write “D-1.” If you’re writing about a day that falls three days after “D-Day,” you’d write “D+3.” 

The Meaning of H-Hour

There’s also an H-Hour. Now that you know what the “D” in D-Day stands for, you can probably guess what the “H” in H-Hour stands for. 

When to Capitalize the 'D' in 'Day'

The “D” in “day” can be capitalized or lowercase. In this article, it is lowercase when it is being used generically, and capitalized when it refers to the 1944 Allied invasion of Normandy during World War II.

Unknown History

For more on the history of D-Day, I recommend you check out the Unknown History podcast from my Quick and Dirty Tips colleague Giles Milton. Season 3 of Unknown History is out now and it’s fantastic. It’s kind of like if the TV show “24” merged with “Saving Private Ryan.” It covers the first 24 hours of D-Day in 12 episodes, sharing stories from fascinating, little-known figures like Guillaume Mercader, a French cyclist spy behind enemy lines, and Axis Sally, an umbrella term for the traitors who broadcast for the Nazis.

Image courtesy of Shutterstock.