Friday the 13th is a relatively new concept of superstition. According to Wikipedia, it rarely appears in writings before the 20th century. Most likely it is made up of two prevailing superstitions, that the number thirteen is unlucky and that Friday is unlucky. The story of the Knights Templar, the protectors of the holy grail, being arrested on Friday, October 13, 1307 really entered the legend way back in…2003 (again, stated in Wikipedia). The Wikipedia entry on the superstition of Friday the 13th has more information. The likelihood of the 13th happening on a Friday is statistically the same as it showing up on any other day (Wikipedia’s article says that it appears on Friday more than any other day, but only by one day in a 400 years cycle). It would stand to reason that the entire month in which Friday the 13th falls would also be unlucky since that month must begin with Sunday the 1st. One question I have heard asked is, "Why do bad things happen on Friday the 13th more than any other day?"
Truthfully, Friday the 13th is no more unlucky than any other day. The above-stated lore behind this particular day/date combination simply makes it stick out more than any other. The idea that more bad things happen on this particular combination could be because people are attuned to bad things on this particular day. When something bad happens on any other day, people would just chalk it up to bad luck, but when something bad happen on Friday the 13th, it’s suddenly the mystical nature of this particular day that caused the bad things to happen. Friday the 13th is a superstition like any other. The person who believes that bad things will happen because of the date on the calendar will experience more bad things.