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What is the significance of the name D-Day? My whole life I thought it was a 4 day plan, starting with A-Day, then B-Day, C-Day and finally D-Day.

I just learned that in French, it's referred to as... Jour J, which just means J-Day. Shatters my theories.

So what is the significance of the name D-Day? Why is it known as 'First letter of the name' Day?

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    Documenting preliminary research will improve both the probability of an answer and the quality of the answer(s). Quick Google search points to the WWII museum; D-Day stands for "Day" It is recursive.
    – MCW
    Dec 21, 2022 at 17:36
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    I tried and I don't know if I was not presenting my query right to google, but it was flooded by 'What is D-Day' results (talking about the operation, and not the name)
    – Fredy31
    Dec 21, 2022 at 17:37
  • In current French, "le jour J" is often used to refer to a project deadline.
    – grahamj42
    Dec 22, 2022 at 12:34
  • @MCW it's definitely not recursive. It's just a "variable" called D, and D represents a day.
    – JakeRobb
    Dec 23, 2022 at 19:11
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    Something similar happens all the time in algebra. You might have a number n, a time t, a distance d Dec 23, 2022 at 20:46

4 Answers 4

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D-Day and H-Hour are generic terms for the beginning of an operation when one does not yet know the date and time of the operation. Thereafter planners refer to the days following the start of the operation as D+1, D+2, D+3. For example, Operation Overlord was tentatively planned for June 5th, but weather delayed its start to June 6th. Because the plans referred to "D-Day" and "D+1", the plans need no adjustment.

The D and H don't mean anything, they're just a bit of jargon to clearly and succinctly indicate the start of the operation. "On the day we will" or "at the start of the operation we will..." vs "On D-Day we will...". It's like in math, "you have X apples and I have X+1 apples", except it's "you have A apples, and I have A+1 apples" to avoid ambiguity about what X+1 means.

At the time, Operation Overlord was just another operation (a big one) which would be talked about in terms of D-Day and H-Hour. It's only the fame of the operation where the term "D-Day" became popular with laymen and attached to that particular operation.

The earliest recorded practice is from WW1 in "Combat Orders" from the The General Service Schools at Fort Leavenworth, Kanas 1922-1923 Section IV.25.

"D DAY" AND "H HOUR."—a. General—(1) When orders or plans are prepared for an operation that is to take place on a date, and at an hour, as yet undetermined, or, concerning which, secrecy is essential, the expressions "D Day" and "H Hour" are used to indicate that the date and hour of the operation are to be announced in subsequent orders. For example, field orders No. 7 state "* * the division attacks on D day at H hour * * *." Subsequent orders state "* * * * reference FO No 7, D day is January 15; H hour is 5:30 AM * * *." It is then understood that the operation ordered by field orders No 7 is to take place at 5:30 AM, January 14.

(2) D Day and H Hour are fixed, the same for all elements of the command. When the operations of any element of the command are to commence at some time prior or subsequent to D Day or H Hour, the time is indicated as "D-Day (or H Hour) plus (or minus) so many days (hours or minutes)." This is explained in detail in sub-paragraphs b and c following.

b. D Day. (1) There is but one "D Day" for all units participating in a given operation. Thus it is erroneous to announce that D Day, for example, is January 15 for some elements of a command, and is January 16 for other elements.

(2) The first, second, third, etc., days, following D Day, are referred to as "D plus 1 day," "D plus 2 days," "D plus 3 days," etc.

(3) The first, second, third, etc., days, preceding D Day, are referred to as "D minus 1 day," "D minus 2 days," "D minus 3 days," etc.

(4) (a) Nights are referred to thus:

  • "* * * night, D day-D plus 1 day, * * *"
  • "* * * night, D minus 1 day-D day, * * *"
  • "* * * night, D plus 1 day-D plus 2 days, * * *"
  • "* * * night, D minus 2 days-D minus 1 day * * *"

(b) For example, assuming D day to be January 14, then the nights referred to, in the above illustration, are:

  • night 14-15 January,
  • night 13-14 January,
  • night 15-16 January, and
  • night 12-13 January.

c. H Hour (1) There is but one "H Hour" for all units participating in a particular operation. Thus it is erroneous to announce that H Hour, for example, is 10:00 AM for some elements of the command and is 2:00 PM for other elements.

(2) H Hour is some specified hour on D Day; i.e., it is included within the twenty-four hours (midnight to midnight) comprising the calendar date of D Day. To state, in an order, that "H Hour is on D Day" is redundant, hence unnecessary. thus:

(3) Specified times, following H Hour, are indicated

  • "* * * at H plus 1 hour * * *"
  • "* * * at H plus 25 minutes * * *" or
  • "* * * at H plus 1 hour and 15 minutes * * *"

(4) Specified times, preceding H Hour, are indicated thus:

  • "* * * at H minus 4 hours * * *"
  • "* * * at H minus 45 minutes * * *" or
  • "* * * at H minus 3 hours and 30 minutes * * *"

(5) Although a combination of hours and minutes is usual, in designating specified times preceding or following H Hour, it is sometimes preferable, in concentration and barrage tables, for example, to use minutes only. In such instances, for example, the time is written "H plus 75 minutes" instead of "H plus 1 hour and 15 minutes."

(6) When indicating an hour that is referred to H Hour as a basis, the time indicated may not be included in D Day. Thus, assuming H Hour to be 12:00 noon, then "H plus 13 hours" is 1:00 AM the following day, and "H minus 13 hours" is 11:00 PM the preceding day. When unnecessary to refer to H hour as a basis, these times are referred to, respectively, as "1:00 AM, D plus 1 day" and "11:00 PM, D minus 1 day."

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    In other words, this is the same thing as "Time T" in scientific simulations.
    – Robert Columbia
    Dec 21, 2022 at 22:38
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    @RobertColumbia And "T-Minus 10, 9, 8...". They also count forward from liftoff, "T-Plus 10 seconds", etc. This is particularly important because a lot of checks have to be done at each second before and after launch. The clock can stop if there's an issue, and people need to know where they are in the checklist when it resumes.
    – Schwern
    Dec 22, 2022 at 1:03
  • Interesting to note (Section IV 24b Time, page 30 of PDF) that the military were still using twelve-hour periods. Wonder when that changed to the, nowadays, common 24-hour periods. Dec 22, 2022 at 6:07
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    Also of interest (to some), this is where the "synchronize your watches" comes from. The Battalion Commander would get his time from a source that was considered authoritative. He would brief his company commanders and they would all change their minute hands so that their watches matched their commanders. Then they would do the same with their platoon leaders, and on down. Later on, watches would have adjustable dials on the ring that could be moved. So if my platoon was to head out at H minus 10, I would ensure my watch time matched my captains and the ring was set to 10 minutes earlier
    – CGCampbell
    Dec 22, 2022 at 11:20
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    @JakeRobb: I would argue that it's less "names that age well", its "technical jargon that escaped into the general populace and gained a non-intended meaning". D-day, in operations planning, is the day any operation happens (but it got appended in the general consciousness to mean the Normandy invasion). I've worked in software deployments that used this nomenclature, e.g. if we turn on this feature on at H-Hour on D-day, we need to have collected data for the prior month to get anything out of it, so we need to deploy at D-31 at the latest, etc. There are many D-days, not all of them are WWII
    – sharur
    Dec 24, 2022 at 22:32
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According to the US Army's Center of Military History (CMH), D-Day is a general military planning term for the day that some operation will start, and the 'D' simply stands for day. This is why the French equivalent is Jour J: 'jour' means 'day', and it's just following the same first letter + word pattern as D-Day.

It's used as a shorthand when talking about timelines without referring to a specific date: D-3 would be three days before an operation starts (day minus 3), D-Day is the day itself (day minus day, i.e. 0), D+3 would be three days after it started etc. Additionally, H-Hour is used to refer to the hour that an operation begins.

The CMH claims the first known use of D-Day and H-Hour by the US Army is from WWI, in Field Order No. 9 issued on September 7, 1918 to the First Army of the American Expeditionary Forces. This field order and others can be found here, and on page 203 of the document/page 215 of the PDF you can find the following:

  1. Mission of the American First Army:

(a) The First Army will attack at H hour on D day with the object of forcing the evacuation of the St-Mihiel salient.

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There doesn't appear to be an answer WWII museum cites two different explanations. Eisenhower's staff backs the interpretation that "D" stands for "Departed Day".

In French, isn't "day" "jour"?

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    This is, knowing french it doesn't cover the Departed theory. Dont have a quick word to translate departed to but I don't think one that starts with J exists. Guess its just the alliteration. Or those who started saying J-Day in french didn't know what D meant and they thought it was just D for Day.
    – Fredy31
    Dec 21, 2022 at 17:47
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    @Fredy31 - The two answers from Giter and Schwern show that D does just stand for "day". Dec 22, 2022 at 13:47
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    This is a good example of how we should be careful of using ex-post facto witness statements as historical sources. For example, we know "any amphibious operation has a ‘departed date’" is wrong because D-Day is not unique to amphibious operations. Eisenhower and a Brigadier General would have known that, so maybe they didn't give the question much thought.
    – Schwern
    Dec 22, 2022 at 22:42
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It likely dates back to the convention established by the 19th century abstinence movement.

Some people called themselves abstainers because they never drank liquor, even though they did occasionally drank wine or beer.

Those that really were abstainers called themselves "Total Abstainers". This got shortened to "Totalers", then "Capital-T Totalers", then "T-Totalers", and finally "teetotalers".

So "D-Day" is simply short for "capital-D Day", the that's so important and unique that it must be spelled with a capital "D".

People are similarly referred to as "capital-C conservatives" or "small-R republicans", to distinguish between personal views and official political parties.

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    The origin of "teetotaler" is disputed. The rationale for D-Day is incorrect; it is not unique to Operation Overlord, it was in use for decades prior.
    – Schwern
    Dec 22, 2022 at 22:32

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