SF urban Riders is hosting a MTB festival. Info and registration can be found here - First Annual SF MTB Festival (http://sfurbanriders.org/wordpress/sf-mtf-festival/)
You are invited to participate in the FIRST Annual San Francisco Mountain Bike Festival, hosted in McLaren Park. A fundraiser for the McLaren Bike Park, the event is FREE to attend with a youth BMX area, beginner MTB riding zones, a professional bike jump demo, and other activities.
For the racers - The headline event is the McLaren Short Track Challenge, a competitive event on a hybrid Cross Country course. The XC race does require a waiver and registration fee to participate. $25 for Beginner Class and $45 for other Classes.