Critical Mass happens on the last friday of every month in cities all over the world. It's a chance for cyclists to ride together through the streets of the city.
The Toronto Critical Mass rides gather at the corner of Spadina and Bloor at about 6pm (the ride departs at 6:30).
The Ottawa Critical Mass rides meet at Confederation park at 5:30pm, and depart at 6:00.
More information can be found on Wikipedia.
Click here to see photos of some of the rides I've been to.