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.