It’s long been a given that American Jews who rarely if ever attend religious services throughout the year come to synagogues in droves on the High Holy Days. To enlarge their sanctuaries, synagogues open movable partitions; to accommodate even greater throngs, they may hold “overflow” services in their social halls, in rented theaters or, when necessary, in large tents. When even these prove insufficient, enterprising rabbis open “mushroom synagogues” that like their namesakes pop up briefly in the early fall and disappear just as quickly.