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.
"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.
(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
- "* * * 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."