I understood that the “D” stood for, “Day,” making the name of the invasion “Day-Day.” But I find that hard to believe. Someone else told me that it stood for “destruction,” but that sounds stupid. Anyone actually know?
I am sure there will be a flood of answers, but I will type fast. The D stands for nothing. In briefings about the invasion it was referred to as taking place “On D day at H hour”, designating an, as yet, unknown day. The D was never meant to be attached to the operation, but it was what stuck.
I would google for it, but this info comes from Stephen Ambrose’s excellent “D-Day”.
fruitbat has the essentials.
In planning for any event that is to take place at a future and yet to be determined date and hour, the hour is lableled “H” and the day “D”.
For example, in the invasion a naval bombardment and air attacks must start some time before the landing which is H hour. So the planners note that naval bombardment starts at H - 1 hours on D day. Air attacks start at H - 2 on D day.
Beach obstacles, such as mines, must be cleared by H - 5. Ships must be loaded on D - 1 and so on and so on working backwards from the actual time of the landings.
Then when the date and hour are actually set, the schedule is already in place and those involved know when their part has to be completed.
Note that every invasion force during WWII had a D-Day when they invaded. Normandy was the biggest, so the name stuck to it, but there were many other D-Days during the war, and choosing the name for the Normandy invasion was just the standard way it was referred to.
In that case, was the day after D Day something like D2 Day or the like? (Making D Day shorthand for “D 1 Day” or “Day 2 D” or somesuch)? I guess there’s no reason to keep using it after the invasion, I would assume the “H Hour” and “D Day” notation was used in planning documents.
No, it would just be “D+2”; the word “day” is not normally used.
Well, the plans probably included objectives to be gained by H + x and D + y but these are naturally tentative and subject to change without a lot of notice depending upon how things go.
The “D” was simply an arbitrary designation for a reference point.
Um, so basically, what everybody else said (I’m late but I brought a cite) 
Copyright me, from my book:
“D Day” The term used in planning for an as yet unspecified date on which military operations will begin. The time operations will begin is called “H Hour.” The first use of this term seems to have been in an operations order issued on 7 September 1918 by the American Expeditionary Force in France. The “D Day” designation is so convenient in military planning that its use has become universal. The most famous “D Day” is 6 June 1944 when the Allies invaded Normandy (OVERLORD). In the post-war era, to avoid confusion with the OVERLORD invasion, some military operations have avoided the “D Day” designation. The Allied invasion of Iraq and Kuwait began on “G Day.”
I would add that ‘M Day’ was used by the French to designate a yet-unknown day of mobilization as far back as the start of WWI.
Different but similar question:
What is Ground Zero?
Do you speak from experience, or is this just a guess? I ask because in Band Of Brothers, they always say “D-Day plus 5”, 6, whatever. I realise an HBO miniseries, well-done though it may be, might not be the best source though, and you may very well know better.
In modern operations, it would be called “D plus two” or “H plus two”.
I believe the convention held true in WWII as well.
This was originally the central point on the ground of an atomic explosion - first popularly used to describe the centre of the destruction at Hiroshima and Nagasaki. I presume it’s a US military term.
It’s also a nickname for the Pentagon.
To be a little more precise, since the bombs exploded in the air, this designated the point on the ground directly below where the bomb exploded.
When I visted the Pentagon about 1995 I was told that the coffee shop in the central courtyard was called the “Ground Zero Cafe.”
Was this because of the assumed (and probably correct) assumption that in the event of a nuclear war there would be a bomb placed as close to the middle as possible?
I wasn’t there, but I used to do archival research on the subject. In all the action reports and other writing produced by the forces at the time, I never saw the usage “D-Day + x”. It was always “D + x”. I should also mention that this was consistent across services (Army vs Navy) and theaters (Pacific vs Europe). I would guess that the producers of the series used the former construction to clarify for a general audience the specific day of the action depicted.
D-day and the like were also popular because of radio communications. Poor quality and unreliable reception meant that single-syllable words might get lost in the static. That’s how the radio alphabet (alpha, bravo, charlie, delta, etc.) developed. Since you weren’t about to refer to “invasion day” in a radio message the enemy pickup, you needed a code. In addition to being standard military terminology, “D-day” was thought to communicate more clearly than simply “the day.”
in W.W.I the British Army used ‘Z Day’ as their equivalent
As posters have already noted, “D” was established military parlance for “Day” when meaning a particular, important day, and “H” stood for “Hour” in the same way.
That people took to using the redundancy “D Day” has to do with semantics or psychology; people found it awkward, apparently, to simply say “D”. In the same way, many people refer to “ATM machines”, when what they are effectively saying is “automated teller machine machines”. I used to work for a military procurement agency which made use of "PAT"s, these being “product assessment teams”. They were referred to invariably as “PAT Teams”.