We’re about to see a liturgical convergence in the Islamic and Jewish calendars. Jews around the world will fast and mourn on Tisha B’Av, a day remembering the destruction of the Temples in Jerusalem. The next day, many Muslims will fast and mourn in observance of Ashura, recalling the death of a revered Muslim forebear in the year 680. The occasions share parallel themes and practices, and they may tempt leaders on both sides to use the convergence as an occasion for joint activities. But Muslim-Jewish relations will be best served if we don’t come together for this commemoration.