Tonight, and every third Friday of the month. From Yelp reviews, it looks like it was pretty crowded at their inaugural event last month.
From the event page at https://upallnight.us/events/undiscoveredsf the food vendor hours are 5pm - 10pm.
http://www.sfweekly.com/dining/undiscovered-sf-a-filipino-night-market-hits-the-old-mint-sept-15/
