Critical Mass has been running in SF since 1992, which makes it one of the longest-running recurring cycling events in the country. It's not a race, not a tour, and not a guided ride — the route is decided in the moment by whoever's at the front. Some months it's a few hundred riders, some months it's pushing a thousand, depending on weather and season. Summer Fridays tend to draw the bigger crowds. The ride typically lasts 90 minutes to two hours and winds through neighborhoods that vary each time.

Practical tip: lock your bike at the plaza before the ride if you're arriving by transit, then use it for the event itself — don't try to drive and park nearby on a Friday evening. Bring a light and a helmet; the ride runs past sunset in fall and winter. There's no food or drink built in, but the end point often clusters near bars and spots in the Mission or SoMa depending on where the route lands.

If you've got two hours: show up at 5:45, catch the energy as the crowd builds, roll with the mass until it disperses, and grab a drink wherever the group lands.