Message from Cal Academy:
Free admission is available for San Francisco residents of each zip code during the designated dates listed below. Visiting adults may bring up to six children for free entry. Proof of residency is required.*
Next Free Weekend: February 28, 29, March 1
Neighborhoods: Castro, Forest Hill, Glen Park, Noe Valley, Parkmerced, Parkside, St. Francis Wood, Sunset
Zip Codes: 94114, 94116, 94122, 94127, 94131, 94132
For free weekends for other zip codes the entire list can be seen here.