Why is the Normandy Invasion called D-Day?

Hello, and thank you for your question.

"D-day" is a military term originating in the First World War which
references "the unnamed day on which an operation or offensive is to
be launched."
  
  Entry for D-day (Dictionary.com)
  http://www.dictionary.com/search?q=D-day

D-Day is also known as the "debarkation day" (June 6, 1944) of the
Normandy Invasion (Operation Overlord) of WWII. Just as D-day
references the commencement of a particular operation, "H-hour" will
reference the exact time that such an operation would start. Other
similar terms are described in these links below.
  
  FAQ: What does the D in D-Day stand for? (Imperial War Museum
Collections)
  http://www.iwm.org.uk/collections/books/bookfaqs7.htm

  D-day (Grolier Online)
  http://gi.grolier.com/wwii/wwii_dday.html


Additional Resources - Normandy Invasion

Normandy: 1944 (Britannica Online)
http://normandy.eb.com/

D-Day, the Normandy Invasion, 6 - 25 June 1944 (Naval Historical
Center)
http://www.history.navy.mil/faqs/faq109-1.htm


Search Strategy:

"Normandy Invasion" d-day
military term d-day


Hope this helps!
-- mother-ga

D simply stands for Day just as H stands for Hour in military
parlance.

Thus, before the Normandy Invasion, D day, H hour was commonly used
for any important military-related event.

As some of the other posts have said, the D is just a countdown the the big Day.
Such as D-2 -- meaning 2 days until the event.

Good Day.

Everybody has offered accurate answers.  To confirm, I also took a
look.

What does the "D" in D-Day Mean? 

The "D" simply stands for "Day". The term was first used in WWI by the
US Army. "We will attack on "H" hour of "D" Day.
Confirmed with US Army also see Ambrose, D-Day CBWII.

The above is from the website: http://www.rain.org/~hickman/dday.html

Best,
Brad-ga

The D as in D-day, stands for "Deliverance". Delivering the french
nation from the agressor, Germany