Critical Mass is a spontaneous bike ride through the streets of downtown Montreal that happens once a month, every month. There are similar rides that occur at about the same time in hundreds of cities around the world; check out the "Critical Mass worldwide" section to get an idea of what it looks like.

[Wikipedia] does a good job of explaining the event's history. Everyone rides for their own reasons; there is no official organizing body for the ride, and thus no official list of good reasons to participate.  

The mechanics of it are fairly simple: riders gather in Phillips Square, then bike off together into the city. By occupying a whole lane of traffic, we create a safe space in which everyone can cycle. 

Critical Mass is a great opportunity to show off your custom or lowrider bike. Lots of participants like to decorate their rides. If you want to blast some tunes, and have a stereo you can strap to your bike or a trailer, go for it. If you're there to support a political or social cause, fly a banner to let everyone know. Remember, it's a party!

Many participants like to print out home-made flyers explaining what the Mass is and what their reasons are for participating, since there's never a shortage of curious passers-by.*

The only sure thing about Critical Mass is when and where it starts. There is no set route; the Mass follows whoever happens to be in front.

*There is apparently a by-law in Montreal that prohibits the distribution of pamphlets, so unless people ask for them, distribute at your own risk.

What Critical Mass is: 

- A spontaneous ride through the streets of Montreal (and hundreds of other cities around the world), without formal leadership or organization

- Open to everyone, regardless of age or ethnicity.

- An opportunity to ride in the city without the fear of motor vehicles

- A celebration of bike culture

- A chance to meet other cyclists in a relaxed, festive atmosphere

- A way to promote a better, more ecological means of urban transportation

- A way to get a glimpse of what a safer, quieter, more cyclist- and pedestrian-friendly city might look like

- Lots of fun!

What Crtitical Mass isn't:

- Violent, dangerous, or exclusive

- A race

- An opportunity buy, sell, or do business  

- An excuse to deliberately anger motorists