Famous line from movie Silent Night, Deadly Night 2, in which the maniac killer randomly kills someone while they were taking out the garbage. Look it up on Youtube.
Also, it could be used to describe something that is completely terrible, negative, or unfortunate. Substitute for "Damn that sucks!"
guy 1: "Ah man! Somebody just stole my damn car!"
guy 2: "GARBAGE DAY!"
guy 1: "Man, that girl that I like just had sex with somebody in my bathroom at MY party!"
guy 2: "GARBAGE DAY!!! What a cunt!!!!"
A day of death (either your own or somebody else's, metaphorically or literally). It can be said as part of trash talk or more fittingly as a war cry or victory cry.
The phrase is usually said without pronoun and is followed by a maniacal (although not hysterical) laugh. It is a reference to a non-intentionally hilarious scene in the shitty 1987 film 'Silent Night, Deadly Night II.'
Dead guy: Dude, you shot me by surprise, although I did have enough time to stand around and shout "huh? no!"
Murderer: GARBAGE DAY! *shoots to kill, then looks at gun with new found admiration* Muha-ha ha-ha-ha.
Scene from a crappy horror movie sequel called "Sillent Night, Deadly Night II." It is unintentionally hilarious because of poor acting and poor writing.
It has become a popular Internet meme, often featured in Youtube Poop videos.
A man is taking out the trash. He picks up a can, and on the other side is the killer.
Killer: GARBAGE DAY!!
Killer: *raises gun*
Man: *is shot and flies backwards, tossing the can and garbage* *moans*
Killer: *laughes manically and twirls gun* *blows smoke off* *laughes again and walks away*