The phrase "only in San Francisco" gets tossed around a lot. But there are very few events that can honestly be described as happening only in San Francisco, and Folsom Street Fair is one of them. It's one of those events, like Pride or the Mission District's artistic take on the Mexican Day of the Dead, that defines the city. It can't be understood by watching YouTube, Channel 5 news reports, or even reading SF Weekly — it must be experienced in the flesh. Folsom is the "granddaddy of all leather events," and the largest fetish festival in the world. This year marks the fair's 30th anniversary, and organizers have gone all-out to celebrate. A performance art stage has been added to the erotic artists area, the elevated steel cage dancers are returning, and the festival's time allotment has been extended to allow for an extra half hour of sin. As always, there will be multiple stages of dance music, public floggings for charity, and enough leather straps to terrify a herd of cattle. Just go.