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D-Day Articles
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What does "D" in D-Day mean?
Armada: Largest Invasion
What does the "D" in D-Day stand for?
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 our planning.

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 after H-Hour)
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 execution.

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 Operation.

I hope this explains what D-Day and H-Hour mean.  If you have any questions, please e-mail me at militaryhistoryonline@hotmail.com.

Brian Williams
www.militaryhistoryonline.com

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