In Boston, there's no mistaking when the circus comes to town: during its annual visit, the Big Apple Circus alights right on City Hall Plaza. This season's show, Bello Is Back! (April 3 - May 16), features the yclept clown, platinum pompadour high-flying as ever -- as will be the Russian trapeze artists and aerialist Regina Dobrovitskaya, who swoops on silk streamers.