Not only is the period from about Labor Day to the end of October the best from a weather standpoint, it is advantageous in that the fall seasons of many cultural institutions (SF Opera, SF Symphony, etc) start up right around Labor Day. If you're sports minded, college football is on at Cal and at Stanford, baseball is still on at SBC Park and at the Oakland Coliseum, and the NFL teams are launching their seasons as well (although both teams are rather hapless at this time).