A massive bike ride, at a casual pace, where hundreds of cyclists take over the streets of Chicago and wish people a "happy Friday!". Tons of fun :)
Chicago Critical Mass starts downtown Chicago, rain or shine, the LAST FRIDAY OF EVERY MONTH at 5:30pm. If you're a wee bit late, don't sweat it (too much) as the Mass often takes a bit of time getting going. The ride gathers in Daley Plaza around the Big Picasso.
More info: http://chicagocriticalmass.org/about