The terms D-day and H-hour are used for the day and hour on
which a combat attack or operation is to be initiated. They
designate the day and hour of the operation when the day and hour
have not yet been determined, or where secrecy is essential. The
letters are derived from the words for which they stand, "D" for
the day of the invasion and "H" for the hour operations actually
begin. There is but one D-day and one H-hour for all units
participating in a given operation. It is unnecessary to state that
H-hour is on D-day.
When used in combination with figures and plus or minus signs,
these terms indicate the length of time preceding or following a
specific action. Thus, H-3 means 3 hours before H-hour, and D+3
means 3 days after D-day. H+75 minutes means H-hour plus 1 hour and
15 minutes.
Planning papers for large-scale operations are made up in detail
long before specific dates are set. Thus, orders are issued for the
various steps to be carried out on the D-day or H-hour minus or
plus a certain number or days, hours, or minutes. At the
appropriate time, a subsequent order is issued that states the
actual day and times.
The earliest use of these terms by the U.S. Army that the Center
of Military History has been able to find was during World War I.
In Field Order Number 9, First Army, American Expeditionary Forces,
dated September 7, 1918: "The First Army will attack at H hour on D
day with the object of forcing the evacuation of the St. Mihiel
Salient."
D-day for the invasion of Normandy was set for June 6, 1944, and
that date has been popularly referred to by the short title
"D-day."
it's just an interview! don't tell me you've even got a strategy or
war plan planned out! the art of war by sun tzu? 
