Shabbat is observed on the seventh day of the week. It begins at sundown on our Friday night, and ends at sundown on our Saturday night. Jews observe the Shabbat (Sabbath) on Saturday. According to Jewish tradition, that means the period of time between Friday sunset and Saturday sunset.
The official sunset time in San Francisco on October 31, 2009, is 6:12 PM. NOTE: California is a big place and sunset time can vary from place to place, depending on where you stand. But it will probably not vary by more than 2-3 minutes from the sunset time in San Francisco.
The question is recorded as having been asked on the 19th of February 2012, but it would depend on your time zone, so it may have been the 18th of February 2012 where you were at the time. The 18th of February 2012 was a Saturday.