What does the "D" in D-Day stand for?
This is the most asked question I get from people via e-mail. In short,
the "D" in D-Day comes from the word "Day" as does H-Hour comes from "Hour".
Military planners use D-Day and H-Hour in every operation to designate "THE" Day and
Hour that an operation is scheduled to 'kick off' or start. Instead of
saying "D+0", we just use "D-Day" because it is easier to say and makes more
sense. Operation Torch, Operation Avalanche and all operations had a
D-Day and H-Hour. In fact, to this day, we still use D-Day and H-Hour in
Why have a D-Day and H-Hour?
Well, when planning an operation of any kind, planners have to come up with a
date and hour on which to start. That way, all units and participants
will know when and where they need to be and what their objectives will
be. For instance, let's say you're the commander of any unit in any
war. As a commander, you'll need to know what your short-term objectives
will be in a certain operation. Days before the assigned D-Day are
notated as "D-1, D-2, etc" (D minus one, D minus 2) and days after the assigned
D-Day are notated as "D+1, D+2, etc" (D plus one, D plus 2).
If we take Operation Overlord as an example (Normandy), you might be given
objectives such as: (these are only imaginary timelines and simplistic)
D-2: Have all units in bivouac areas
D-Day, H+3: Land on Omaha Beach (this means your unit will land 3 hours
D-Day, H+5: Occupy Vierville
D-Day, H+13: Occupy St. Lo
D+1, H+3: Occupy Caen
D+14: Occupy Le Havre
D+60: Occupy Paris
Long-term objectives (such as occupying Paris) might be included at the
strategic level, but of course would be adjusted as the operation
progressed. You can see that the unit will know where it should be and
what it should be doing (at least in the near future of operations). It
allows commanders to prepare and get ready ahead of time for the upcoming
operation. It also lets everyone know what the overall plan will be
and everyone's involvement in it.
You can also see that there will many D-Days and H-Hours within a Theater of
Operations. For instance, the Breakout of Normandy would have it's own
D-Day and H-Hour with preliminary bombing and special operations that kick off
before the actual attack. The attack on Cherbourg, the attack Paris,
etc., etc. And, of course, it would have the D+X timeline to follow.
Also, remember that H-Hour will always be in Greenwich Mean Time (GMT).
This is called "Zulu" or "Z" in military terms. So, H-Hour might start at
0600Z (6 o'clock in the morning Zulu Time (GMT)). Zulu (GMT) is used so
everyone in all the timezones will be synchronized in their planning and
If "D-Day" is a generic term, why has it come to mean Operation
Overlord's (Normandy) D-Day?
I would suspect that the reason for this is because it was the largest and is
the most famous American "D-Day" to date. It has become the de-facto
"D-Day" even though it is a generic term and every operation has a "D-Day".
Do we still use D-Day in current operations?
Yes, we do. We still use the terms D-Day and H-Hour to designate the
time of a certain event and start of operations. Operation Desert Storm
had a D-Day and H-Hour as did Operation Iraqi Freedom and as does every
I hope this explains what D-Day and H-Hour mean. If you have any
questions, please e-mail me at