Although Critical Mass is not unique to Chicago, nor did it start in Chicago (it originated in San Fransisco) it is something I find special about Chicago, as not all big cities have it, and some big cities try to have it and have little success.
Critical Mass is a bike ride that take place the last Friday of every month. Bikers meet en mass at the Daily Center starting around 5:00 at night. The rout is never disclosed until just before the ride begins, when people will hand out maps. This is a new development, the rout used to be totally secret.
The ride begins when the bikers begin circling the Daily Center. The speed of the ride is very slow, so you will see anyone from little kids to seniors to experienced bike messengers. Everyone rides, and it is a friendly and cheerful experience. The cars don't always appreciate the sometimes thousands of bikers holding up traffic, but most people respond positively to the bikers smiles and often return the "Happy Friday" greeting being yelled out.
Originally started to spread a message from bikers to cars, that the road must be shared, it is now banned in New York City because of how badly traffic gets messed up. Here in Chicago, thanks to our bicycling enthusiastic mayor, the mass is actually accompanied by biker cops, who help block traffic. Despite this, the message doesn't always seem positive, as cars and bikes do often get upset with each other during the ride becasue the cars are forced to wait as all the bikes pass.
Overall though, the ride is a very happy and fun event, so check it out, the last Friday of the month (although probably not this one as if falls on Christmas and the Winter months tend to be pretty low in attendance, so I'd say wait until May or June and then you'll get the really great experience).