For a week during the holidays this winter, Michelle Jasny left the Island for a camp in the Catskills. Armed with her accordion, she joined some 300 people at KlezKamp, a weeklong retreat for klezmer musicians. Though she’d been playing for nearly a year, and also has a background in piano and guitar, Ms. Jasny was blown away by the intensity of the repertoire, and the music. “Some of this is very fast,” she said of the traditional Yiddish music. “We were playing like nine, 10 hours a day, so it was very intense.”