Having hair growing between the eyebrows. Here it is deemed unlucky to meet a person thus marked, especially if the first one meets in the morning. Elsewhere it is a favourable omen. The term, I suppose, had been primarily applied to a woman as by this exuberance indicating something of a masculine character.—John Jamieson's Etymological Scottish Dictionary, 1808
Among the Egyptians and our British ancestors, the 14th of May was the most unlucky day. The day of the week upon which that fell was deemed the most unfortunate, and nothing of consequence was done upon it. After the introduction of Christianity, Childermas Day took the place of the 14th of May, the superstition being transferred thither.—Thomas Fosbroke's Encyclopedia of Antiquities, 1843
Robert Nares' Glossary of the Works of English Authors (1859) contained the curious expression, "honest as the skin between the brows," probably a reference to the above-mentioned superstition, which is found in Shakespeare's Much Ado About Nothing and elsewhere.
So, if you can change which day is unlucky simply by fiat, why not assign it to February 29th so it only shows up every four years? Or for that matter, why not set it for Octember the 300th?