“Thirteen is a famously unlucky number in the Western world. I certainly grew up with the belief that to invite thirteen guests to sit around the table doomed the last to some nameless dread–so, to avoid that fate, out table was always laid to include fourteen. It was a belief shared by Napoleon, F.D. Roosevelt and John Paul Getty, and concern over the number 13 is the most common form of Western superstition. Hotels often have no room 13, tower blocks tend to avoid a 13th floor, and travel agents know that the thirteenth of a month (especially if it falls on a Friday) will be short of bookings.
The most common explanation of unlucky thirteen is the Last Supper, where thirteen sat down to eat, one of whom was a traitor plotting the arrest and judicial murder of his host and master. But similar stories can be found in many other cultures, such as the Viking Norse, who remembered how Loki stumbled into a gathering of twelve gods (from which he had been excluded) and in his envy started plotting the events that would lead to the end of the world.
Robert Graves enthusiastically listed in The White Goddess the various mythological companies of thirteen that tend to lead to the betrayal, if not sacrificial death, of one of their members: be they Arthur and his twelve nights, Odysseus and his twelve companions, Romulus and the twelve shepherds, Roland and the twelve peers of France, Jacob and his twelve sons, of Danish Hrolf and his twelve Berserks. Not to mention the thirteen dismembered portions of Osiris’s body recovered by Isis from the Nile.
The ultimate cause of our attitude to thirteen may be that the thirteenth month of the year was always weak and withered. For, although twelve lunar months almost fill up our solar year (to produce 360 days from twelve sets of 29 and a half days), there was always the issue a left-over period of five days. This was considered in ancient cultures to be the thirteenth month, a five-day oddity, often believed to be a period of immensely bad luck where the world was not policed by the normal powers, and evil spirits held brief reign. Some cultures made this into a Saturnalia-like carnival, where the norman roles of society were reversed; others deemed it a needful time for sacrifice.”
Excerpted from: Rogerson, Barnaby. Rogerson’s Book of Numbers: The Culture of Numbers–from 1,001 Nights to the Seven Wonders of the World. New York: Picador, 2013.