Actually, Folsom St Fair is only for one day. Always the last Sunday in September. Castro St fair is much more a regular street fair.

Funny story from when I went to FSF a few years ago. Fair closes at 6pm, so we went into a bar to sit down and chill for a while. Friend goes outside to smoke, and observes two guys having sex (with private parts exposed). A police officer approaches them, and one of the guys says to the officer that because it is fair day they can have sex in the street if they want to. The police officer says "You can do whatever you want with your boyfriend, but it is after 6, the fair is closed, and you can't have any open achohol on the street." and made them dump their beer.