The number 13 as well as the sixth day of the week have similar reputations said to date as far back as ancient times.
It seems their inevitable conjunction from one to three times a year, three of these occurrences in 2012 and exactly 13 weeks apart portends misfortune. According to some sources it's the most widespread US superstition.
Some refuse to work on Friday the 13th; some won't visit restaurants; others wouldn't think of setting a special event on this date.
The term paraskevidekatriaphobia may be an issue to as many as 21 million. This is eight percent of Americans who believe this ancient times superstition.