There are witches among us. Halloween approaches, signaling the time of year when dentists crack their knuckles with glee and Amazon overflows with sexy versions of otherwise reasonable costumes. But Samhain (pronounced sow-ain) is the original Halloween, a modern(ish) version of an ancient Gaelic end-of-harvest festival. Co-opted by the Catholic Church as “All Hallows’ Day” or “All Saints’ Day,” Samhain lands midway between the fall equinox and the winter solstice, celebrated on the evening of Oct. 31 and into Nov. 1. Much of modern Halloween is about demons, monsters and cult serial killers, none of which have anything to do...