It appears my friends in Nairobi got it all wrong with the dates, but that doesn’t matter. Wiki has this to say about CM
Critical Mass is a cycling event typically held on the last Friday of every month; its purpose is not usually formalized beyond the direct action of meeting at a set location and time and traveling as a group through city or town streets on bikes.
We hold ours last Saturday of the month with an aim to create awareness to the city’s bad drivers that we exist, that it is possible to share the road.