D-Day is a term often used in military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

 parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable
Variable (mathematics)
In mathematics, a variable is a value that may change within the scope of a given problem or set of operations. In contrast, a constant is a value that remains unchanged, though often unknown or undetermined. The concepts of constants and variables are fundamental to many areas of mathematics and...

, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours
Military designation of days and hours
The military designation of days and hours within the North Atlantic Treaty Organisation , is specified in AAP-6 , NATO Glossary of Terms and Definitions, and marked in what follows...

 for similar terms. The initial D in D-Day has had various meanings in the past, while more recently it has obtained the connotation of "Day" itself, thereby creating the phrase "Day-Day", or "Day of Days".