Unfazed by the bombs, undeterred by the warnings, and in the face of the raging conflict, more than 35,000 ultra-Orthodox Jews from across the world have journeyed to Uman, Ukraine, to celebrate Rosh Hashanah, the Jewish new year. “Going to celebrate in a war zone en masse is crazy,” said Azoulay Ruben, a 22-year-old trainee dentist from Paris. “But at the same time, it’s a beautiful thing.” Rosh Hashanah is a two-day holiday that falls in September or October, marking the beginning of the high holy days. In Israel, it is usually celebrated with family visits and food: traditionally, pomegranates...