Yesterday I cycled in the Edinburgh Critical Mass.
The idea about Critical Mass is to raise awareness of cycling. A large group of cyclists just get together and cycle around a city as a group. It is completely unorganised except the starting location and time. No route is planned in advance. Anyone in the group can decide to take the lead and the route is chosen on the fly by consensus.
In Edinburgh, on the last Friday of every month at 17:30 for 18:00 set off, a bunch of cyclists get together at the the foot of the mound beside the National Galleries of Scotland. The cycle takes around an hour or two.