Friday the 13th
For horror fans of a certain age, Friday the 13th will forever evoke images of our favorite hockey mask-wearing monster tearing through a cabin full of lascivious teenagers. But for many people the day is simply one of bad luck. The unlucky holiday wasn't popularized until the 19th century, and there is no firm agreement on its origin.
Some believe that its negative reputation stems from the number of apostles present at Jesus' last supper (there were thirteen) and the belief that he was crucified on a Friday, while others think it has to do with the date Philip IV of France arrested hundreds of Knights Templar (Friday, October 13th, 1307). A more secular perspective states that the number thirteen itself is considered unlucky because it's one higher than twelve--a number said to represent perfection since it can be seen in the twelve months of the year, two twelve-hour halves of the day, and the twelve signs of the zodiac. That doesn't explain the importance of Friday, though, which is almost certainly considered relevant for one of the date-related reasons already stated.
There are, of course, conflicting reports on the actuality of the day's misfortune. A 1993 study in the British Journal of Medicine found that there "is a significant level of traffic-related incidences on Friday the 13th as opposed to a random day, such as Friday the 6th, in the UK." Yet a 2008 study by the Dutch Centre for Insurance Statistics found that "fewer accidents and reports of fire and theft occur when the 13th of the month falls on a Friday than on other Fridays, because people are preventatively more careful or just stay home." So there you have it, Friday the 13th either is or is not far more dangerous, or much safer, than any other day of the year, depending on your nationality.