Critical Mass is a movement that began in San Francisco on the last Friday of September in 1992. The tradition has continued with a monthly Friday bike ride and has since spread to cities worldwide.
Organizers say Critical Mass is not only a time to enjoy urban spaces via bicycle, but also to "challenge the use of city streets and the domination of cars and oil in our transportation system."
The weeklong celebration, which began Sunday, will continue today with the opening of a welcome center at 1 p.m. and an art exhibition at 6 p.m., both at 518 Valencia St.
Other anniversary events this week include a screening of a short film, "Critical Mass at 20", a book release party for "Shift Happens: Critical Mass at 20," a game of bike polo, and a concert.
The main event will be a mass bike ride starting at 6 p.m. Friday at Justin Herman Plaza. A special "Kidical Mass" contingent is set to take part in the ride, as well as a women's contingent organized by a female cycling group.
Motorists are advised to expect delays in the area on Friday evening.
A full schedule of this week's Critical Mass events is available at http://www.sfcriticalmass.org/20th-anniversary/calendar-of-events/.