The Olive Festival, which celebrates the local olive harvest, takes place in early December around Sonoma Plaza and in Glen Ellen. It begins with the blessing of the olives in the Mission chapel and includes food and wine tasting at the Barracks.
The Spanish missionaries who built the 21 California missions (from San Diego in the south to Sonoma in the north) planted olive groves because the trees were so useful. The fruit was eaten, and the oil was used for sacramental, cooking, and medicinal purposes.
I found this fact interesting: This variety of olive is no longer found in Spain, so the missions actually helped save it from extinction. It is now being propagated by historic preservation groups so that it will never die out.