Cartman watches one of the Aliens films on TV and we see a snippet of the film where someone says something like, “They mostly come at night, mostly.” And then he uses the word “mostly” in as many sentences as he can. IIRC it was “mostly” 
Any chance of a name or episode number?
Cheers
Pushkin
The name was Cat Orgy. Number 307.
I think this is what you are referring to:
Season 3, episode 307: Cat Orgy
Quote is: “They come mostly at night…mostly.”
http://www.southparkstudios.com/down/download.html?file=/media/sounds/307/CARTMAN_MOSTLY.wav
Yeah, that was one of the ‘triad’. There were three episodes which were supposed to have all taken place on the same night.
Kyle & Kenny are at Jew-Scouts in their’s. The one where the Scoutmaster tries to release Hyman, the evil, um, something.
Stan was at the meteor shower party with his parents in his (the one where somehow the ATF thinks they’re all going to commit suicide).
And Cartman got stuck home being babysat by Stan’s sister Shelly.
I got one for you guys:
What’s the episode where Cartman tells the family that he has found their lost son, and they show up and have butts for faces and he can’t laugh anymore because he has seen the funniest thing he WILL ever see? Been looking for it for awhile
That would be season 5, episode 10 called How to Eat With Your Butt. His description was great, “blonde hair, a brown eye and full rosy cheeks.”
Yeah, and Moses turns up as the Master Control Programme from Tron demanding macaroni and glitter pictures 
;j
Classic. I especially love Moses’ return in the ep “Super Best Friends”.
Jesus: Moses!
Moses: What?
J: How do you defeat a giant stone Abe Lincoln?
M: Um… uh… with a giant stone John Wilkes Booth?
Didn’t Cartman also utter that “… mostly” line at the beginning of “Grey Dawn”, where the townspeople attended a funeral service at the Farmer’s Market?
I’m going to go out on a limb here and point out that “They mostly come at night… mostly…” is a reference to the orphaned waif’s line in Aliens, even though I’m fairly certain that the reason it hasn’t been mentioned here yet is because it’s assumed knowledge.
Oh, right, and because I somehow missed the first line of the OP. Right.