Adding to her informative post is a local weather link to further assist you:
The page has temperature, humidity, etc., as well as an animated fog map, weekly forecast and a bay area map that shows the range of temperatures in areas around San Francisco.
Those with smart phones can retrieve SFGate's weather link to anticipate conditions.
Particular to flingers will be how much hotter than San Francisco it can be in the south bay where we will be visiting Filoli, Sunset, etc. A 30 degree range is not unusual.
Another thing to keep in mind are local garden microclimates. Our dinner on Friday night will be held at the Conservatory of Flowers. After we take our group photo (most likely on the steps of the conservatory), cocktails will be served in one of the tropical exhibits. Plan for a bit of warmth and humidity amongst the foliage there, and keep your eye out for wet floors on grated surfaces. Dinner however, will be served behind the conservatory, under a tent that while unheated, will most likely warm up amongst the body heat of fling participants.
A fancier dinner than ours occurred at the re-opening of the conservatory back in 2003. Perhaps that article will inspire some of you to bring more formal attire... but most likely not. Besides, I think the catering staff have packed away their expensive china and linens by now.
So, best to wear comfortable layers and sensible shoes. Fascinators optional.