Don't be fooled into thinking that San Francisco lives up to the image that you may have of sunny California, and come prepared with nothing but T-shirts and shorts. Okay, so you probably wouldn't do that for a wedding, but we think you get what we mean.
The important thing to keep in mind is that the weather can be quite variable, especially between day and night, and it's best to dress in layers. The typical summer weather pattern is for it to be warm (not hot) during the day, but sometime in the night the fog rolls in and it cools down considerably. It is very unusual to not need a jacket at night, even in the summer. And what you generally think of as summer isn't our warmest time of the year -- that's in September and October.
The weather can also vary considerably over very short distances. It could be 10°F (5°C) warmer in Berkeley and Oakland than across the bay in San Francisco, and another similar jump warmer over the hills to Walnut Creek.
And we've had some weird unseasonal weather lately too -- it was actually hot a few weeks before the wedding, and it rained a little bit the week before. (We're keeping our fingers crossed that there won't be a repeat occurrence.)
While we're hoping for sun, if we do get it you might be happier if you bring sunglasses to the ceremony, as the direction you'll be facing is somewhat towards the sun, and the ceremony is late in the afternoon.
Weather junkies may wish to be informed of the following factoids regarding the wedding day weather in Walnut Creek:
Sunrise 5:49 AM PDT Avg. High 80°F (27°C) Record High 104°F (40°C) (1984) Sunset 8:22 PM PDT Avg. Low 51°F (11°C) Record Low 43°F (6°C) (1976)