It must be Tuesday in the comic.
How do you know?
In real-Northampton, it being a small city, there aren't that many clubs - one of them (which is likely also "that gay nightclub down on Pleasant Street," even though it's only sort-of gay) holds a goth-themed Tuesday, and that's the only one I know, and presumably the only one he knows. Of course, neither living in Northampton nor being especially gothic or much of a club hopper, I wouldn't know; moreover, this is not real-Northampton, this is funny-robot-allosaurus-world-Northampton, where there are all the clubs Jeph needs there to be.
I'm a girl. But yeah, pretty much what you said. Diva's (which holds drag queen nights, so is a little more than sort of gay

) turns into the Haven on Tuesdays. I wasn't really being serious but I had just gotten through reading a comic timeline guessing thread, so I felt like throwing that out there. I don't know of any other goth clubs.