The number 13 has long been considered unlucky. In numerology, 12 is considered complete (12 hours, months, zodiac signs) - and 13 is irregular. There were also 13 guests at The Last Supper (the 13th being that traitor Judas). Jesus was also crucified on a Friday.
A Dutch study from 2008 apparently showed that fewer accidents and reports of fire occurred on the date as people were generally much more cautious.