Also, it could be used to describe something that is completely terrible, negative, or unfortunate. Substitute for "Damn that sucks!"
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!!!!"
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.'
Murderer: GARBAGE DAY! *shoots to kill, then looks at gun with new found admiration* Muha-ha ha-ha-ha.
It has become a popular Internet meme, often featured in Youtube Poop videos.
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*