War, bloodshed, and a cappella music seem unlikely companions in a Christmas show, but All Is Calm is a holiday production without parallel. Subtitled The Christmas Truce of 1914, it revisits in letters and songs of the period a Christmas Eve in World War I when soldiers in the trenches suspended their enmity and joined one another for a night of celebration in no-man’s-land. The story has been told before, notably in Joyeux Noël, France’s 2005 Oscar nominee for best foreign film.