It actually means Day day in army talk, like h-hour. No joke.

A day in which two individuals partake in sexual activity

In other words

D-day: a day to lay down some d

Expression used in online discussion forums relating to mental illness. Originally referred to the time that depression set in, later used conversely by survivors to indicate when their recovery was complete. This confusion has seen it fall out of favor in many circles.

Derived from "d-day", the storming of Normandy on June 6, 1943, and the fact that the word "depression" is often abbreviated to "D" in such forums, as people suffering from "D" have little energy and typing the whole word could be tiring.

A badass, who also gets chicks.

Someone who you do not want to mess with, or fight, but at the same time is a 'ladies man'. Compared closely to Shaft or James Bond.

It’s a day where you just want to fuck, sleep, smoke and eat pussy

When you charge at a girls pussy whilst other man ejaculate towards you and you try to get it in.