Young American Jews are flocking to pop-up events that fill a need for casual, inventive gatherings, while traditional institutions struggle to catch up. New York’s Jews are finding creative new ways to connect with Judaism independent of synagogues – from musical Shabbats for young families in an upscale Brooklyn condo building to pot luck suppers and lots of singing with hundreds of 20- and 30-somethings around the corner; and from a monthly Ecstatic Mincha that pairs dancing with prayer to a private Kol Nidre service for Russian families on the Upper East Side. These and countless other one-off and occasional events are part of a burgeoning wave of gatherings that, much like the pop-up boutiques in vogue in recent years, generate buzz and create impromptu communities. The Jewish equivalent is not a movement, per se, since there is no coordinating body, but an important trend from which synagogue leaders must learn, experts say.