What is Critical Mass?
Critical Mass is a mass bicycle ride that takes place on the last Friday of each month in cities around the world. Everyone is invited! No one is in charge! Bring your bike!

http://www.sfcriticalmass.org/

Q: How can I get involved?

A: Show up! The San Francisco Critical Mass meets on the last Friday of each month at 5:30pm at Pee Wee Herman Plaza at the foot of Market Street. Beyond that, if you have ideas to share you are invited to participate in Critical Mass’s flourishing Xerocracy, in which everyone is invited to make flyers, stickers, zines or other printed material to share their ideas.

Held in San Francisco of 2010, celebrating the new year.