It is a day where everyone takes a day off to throw a gigantic, medium-sized, and/or small LAN party for what-ever game they desire.
This day can be used for the sad excuse of staying home for the release of Halo 2, and/or playing it with a house-hold of friends in a LAN-party ordeal.
The origin of the term is from a comic of Ctrl + Alt + Del.
"I need the time off for obervance. It's a religious holiday."
"Oh yeah? What Religion? What Holiday?"