I think it's once a month, but a bunch of riders get together (the ones here can be several hundred) and ride through the city to promote bike awareness. Cities all over the country have CM events. When I went, we all went to a pub afterwards. In theory, I really liked it. In practice, not so much. I think it just depends on how the tone is set for the ride.
Critical Mass can be positive if you follow traffic law, but it can also be pretty nasty. Check out the Seattle news papers (seattletimes.com, or seattlepi.com) for news about a recent CM ride we had that went very sour. So long as the riders are promoting bike awareness and working with cars rather than taking over the streets, I think it can work. I rode in one once though and that wasn't really the attitude. Good luck!
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