Many of you may think this is a silly post, but other readers will have no idea what the term D-Day really means. Before I post one answer though, let me reiterate on this forum that D-Day just doesn't mean June 6th, 1944, but any invasion such as Southern France, Salerno, North Africa, Tinnian, Guadacanal, etc. There wasn't ONE D-Day, there were dozens and dozens of D-Days. The great OZ has spoken! 
===================
The Meaning of the "D"
Ever since June 6, 1944, people have been asking what the "D" in "D-Day" means. Does it stand for "decision?" The day that 150,000 Allied soldiers landed on the shores of Normandy was certainly decisive. And with ships, landing craft and planes leaving port by the tens of thousands for a hostile shore, it is no wonder that some would call it "disembarkation" or "departed."
There is not much agreement on the issue. But the most ordinary and likely of explanations is the one offered by the U.S. Army in their published manuals. The Army began using the codes "H-hour" and "D-day" during World War I to indicate the time or date of an operation's start. Military planners would write of events planned to occur on "H-hour" or "D-day" -- long before the actual dates and times of the operations would be known, or in order to keep plans secret. And so the "D" may simply refer to the "day" of invasion.
===============
What is the meaning of D-Day?
A lot of people ask me what does D-Day actually means. Well, every operation has it's D-Day. This is the day an operation starts. For operation Overlord this was June 6th 1944, for operation Market Garden this was 17 September 1944, and so one. Instead of just saying, Day, they repeat the first letter to emphasize that this is the day that the operation will start, not on any other day, but this day! The same counts for H-Hour, this is the time the operation will start. Like for the invasion in Normandy, H-Hour for Omaha Beach was 06.30h. In French they say "le Jour-J" for D-Day, In Dutch they say "het Uur-U" for H-Hour, according to the same principle.
But since the invasion in Normandy was one of the most important in WW II, and also because it's the most popular, and one of the most fascinating events from WWII, the operation has gone into history as D-Day. One of many of the history errors. The actual name for the invasion is Operation Overlord.
There are some people who say that D-Day stands for Decision Day, but that's something someone invented later. Why? I guess it sounds good. But that's not the military meaning of D-Day.
===============
D-Day
Origin: 1918
The Yanks were coming! In April 1917, after three years of the Great War that was consuming Europe, we abandoned our neutrality and declared war on Germany, the country we saw as the aggressor. By June of that year, the first units of the American Expeditionary Force reached the battlefields of France. The A.E.F. provided much assistance to the French and British, and victory was achieved and an armistice declared the next year, on the eleventh day of the eleventh month of 1918.
Because we were last on the battlefield, most of the military vocabulary added to our language during World War I came from the British, like the name of the new military vehicle the tank. But apparently it was the Americans who contributed a technical term that would become famous a quarter of a century later, during World War II. Field Order No. 8 from the First Army of the A.E.F., of September 7, 1918, begins, "the First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of the St. Mihiel salient."
The British used zero hour in discussion of an operation whose time was not yet determined or to be kept secret, but Americans preferred H-Hour and D-Day. In the next World War, with American general Dwight Eisenhower in supreme command, the term for the first day of the Allied invasion of France, June 6, 1944, was the American D-Day. That D-Day was and is so famous that it successfully invaded our general vocabulary to become a term that can refer to any day of action or decision. Other letters were combined with Day to give similar emphasis to a particular day, such as V-E Day and V-J Day for the 1945 endings of the war in Europe and Japan, respectively, and E Day for "Education Day" (also attested as early as 1945).
============================
D-Day
In U.S. military history, any designated day for the commencement of a major operation. The designation apparently originated in World War I; its meaning is uncertain, though it is probable that the "D" stands for nothing more than "Day." (The designated time for commencement of action on any D-Day was referred to as H-Hour.) The most celebrated D-Day occurred on June 6, 1944, the first day of the Anglo-American invasion of Europe in World War II.
=====================
Meaning of D-Day
by Brian Williams
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