Besides a gangster film, what’s the ideal place to see ladies with hand fans, long cigarette holders, fringe dresses, feather boas, elbow-length gloves, and beaded necklaces, alongside gents decked out in spats, wingtip shoes, vests, and zoot suits? Well, you go to . . . whatever the location happens to be for the now-semi-annual Shanghai Mermaid party. Much like the Prohibition-era speakeasies, the party is kept on the down-low, shuttling to a secret venue each time (usually at locales in Brooklyn), with little info on the website. But if you do find it, you’ll enjoy not just the era costumes but also the burlesque acts, magic, acrobatics, ragtime music, period-era blues and jazz, and gin martinis. Get there early, because word has gotten around, and hundreds of people show up. But don’t forget your fishnet stockings and beret.