Looks like its free on the weekends according to the bart website
Daily Fee Parking
Customers parking at BART Stations located at Ashby, Colma, Daly City, Dublin/Pleasanton, Fruitvale, Lafayette, Lake Merritt, MacArthur, North Berkeley, Orinda, Rockridge, Walnut Creek and West Oakland, El Cerrito Plaza and Fremont will be required to pay a
daily or monthly parking fee. The fee will apply to all parking spaces in these lots. Daily parking fees are required Monday through Friday from 4:00 a.m. to 3:00 p.m.
http://bart.gov/guide/parking/overview.asp