San Francisco is not a city short on antiques fairs. For starters, there's old faithful in Alameda and the new upstart at Candlestick Park, both of which occur monthly. And if you attend just those two with any regularity, you know how quickly you can become addicted to going. So what's an antique addict to do on the off-weeks between events? This weekend offers just the ticket, with the annual 20th Century Modernism Show & Sale pulling into Herbst Pavillion in Fort Mason.