Brewer's: Thirteen Unlucky
The Turks so dislike the number that the word is almost expunged from their vocabulary. The Italians never use it in making up the numbers of their lotteries. In Paris no house bears the number, and persons, called Quartorziennes (q.v.), are reserved to make a fourteenth at dinner parties.
Jamais on ne devrait Se Mettre a table treize, Mais douze c'est parfait.
La Mascotte (an opera), i. 5.
Sitting down thirteen at dinner, in old Norse mythology, was deemed unlucky, because at a banquet in the Valhalla, Loki once intruded, making thirteen guests, and Baldur was slain.
In Christian countries the superstition was confirmed by the Last Supper of Christ and His twelve apostles, but the superstition itself is much anterior to Christianity.
Twelve at a dinner table, supposing one sits at the head of the table and one at the bottom, gives a party to these two, provided a couple is divided; but thirteen, like any other odd number, is a unicorn.