Because it's been well publicized before, this is just a reminder of the Sept. 29 Cal Performances Free-For-All. Also, briefly, San Francisco museums all have free days, and there is a good comprehensive guide to that.
Note the Yerba Buena Family Day on Sept. 15, from 11 a.m. to 4 p.m.: four museums (3 1/2, actually), three stages, a big block party. Visitors get in free that day to the Contemporary Jewish Museum, the Museum of the African Diaspora, Children’s Creativity Museum, and get to see SFMOMA On the Go (while the museum remains closed for reconstruction/expansion).
A highlight of the outdoor concert on the Esplanade Stage, beginning at 1 p.m., features 2013 Grammy winner Lila Downs and her band, reinventors of traditional Mexican music, fusing original compositions with blues, jazz, soul, and more.