(Undated) --Today is Friday the 13th. It's an unlucky day for superstitious people, though researchers are not sure why. Professors who study the origins of superstitions have found no mentions of Friday the 13th before the 19th century. The experts tell "USA Today" that fear of the day is likely rooted in Christianity, because Jesus was crucified on a Friday and 13 guests were supposedly at the Last Supper. Fear of the day may be unfounded--a study of myths found no link between Friday the 13th and an increase in emergency room visits.