"Legend credits St. Dunstan with having given the horseshoe, hung above a house door, special power against evil.
According to tradition, Dunstan, a blacksmith by trade who would become Archbishop of Canterbury in A.D. 959, was approached one day by a man who asked that horseshoes be attached to his own feet, suspiciously cloven. Dunstan immediately recognized the customer as Satan and explained that to perform the service he would have to shackle the man to the wall. The saint deliberately made the job so excruciatingly painful that the bound devil repeatedly begged for mercy. Dunstan refused to release him until he gave his solemn oath never to enter a house where a horseshoe was displayed above the door.
From the birth of that tale in the 10th century, Christians held the horseshoe in high esteem, placing it first above a doorframe and later moving it down to middoor, where it served the dual function of talisman and door knocker. Hence the origin of the horseshoe-shaped knocker.…The horseshoe could not be hung just any way. It had to be positioned with points upward, lest its luck run out.
Christians once celebrated St. Dunstan's feast day, May 19, with games of horseshoes.…A popular Irish incantation against evil and illness (originating with the St. Dunstan legend) went: ‘Father, Son and Holy Ghost, Nail the devil to a post.'"
