There are free introductory lessons in milonga, tango & vals (tango waltz) starting at 2pm. And performances...
Boombox milonga to follows in the stage area at 3pm. DJ dancing until the batteries did give out on the boom box.
This is a part of San Francisco Parks & Trust's "Jewels in the Square" program for Union Square Park. (http://www.unionsquarepark.us )