The critical mass ride date is the last Friday of the month (usually at 6pm). That is the one thing that is reliable and does not need to be changed so that those who can plan ahead or make it on that day can expect to go. This will be mentioned on the flyers and the route agreed upon at the time of each ride. Though there is no one "in charge", someone who is familiar with the area could advise on a route.
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RE: Critical Mass is an event typically held on the last Friday of every month in cities around the world where bicyclists and other self-propelled commuters take to the streets en masse. While the ride was originally founded with the idea of drawing attention to how unfriendly the city was to bicyclists,[1] the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city or town streets. (from http://en.wikipedia.org/wiki/Critical_Mass)
-Paul