The mustard festival lasting a few months in winter early spring has been the most notable event, but finally a harvest festival will be coming to Napa. This really needs to expand into a community event that spans multiple locations and lasts several weeks. For now there is one Harvest Stomp event, being held at Round Pond Estate in Rutherford on September 6th, 2008.
I think it may be something bigger in the future as it is not a marketing event by one winery, but rather an event organized by the Napa Valley Grapegrowers.