According to National Geographic Fact Book: Of all superstitions, fear of the number 13, called triskaidekaphobia, is the one with the most influence still in the modern world. More than 80 percent of high-rises lack a 13th floor; many hospitals have no room number 13; France and Italy rarely have 13 as a house address; airports often skip gate 13 and airplanes don’t include a 13th row. Most telling, up to $900 million is lost every Friday the 13th because of people not flying or conducting business on this double-whammy day.
Folklorists trace this numeric nervousness to a pre-Christian Norse myth. Twelve gods were having a dinner party in Valhalla, their heaven. An uninvited 13th guest named Loki arrives. A known mischief-maker, Loki then arranges for Hoder, the god of darkness, to shoot Balder the Beautiful, god of joy, with a mistletoe-tipped arrow. Balder dies and the Earth is shrouded in darkness.
A sense of foreboding and doom has attended the number 13 ever since. The Christian story of the Last Supper fit readily into the Norse framework. Judas is the last disciple to show up, making 13 at table; the next day he betrays Jesus. In ancient Rome, covens were reputedly made of 12 witches; the 13th was the devil.