One theory: 13 is an unlucky number and Friday an unlucky day (considered so since The Canterbury Tales, unlucky to start a journey on a Friday).
Note: Crazy to think of Friday as an unlucky day. I love Fridays! End of a school/work week, prepping for weekend, etc.
Another theory: 12 is considered the number of completeness (months, Zodiac, apostles), and therefore 13 is irregular. I found mention of a Norse Myth that 13 seated at a table would result in death... consider the Last Supper - the 13th guest, Judas, betrayed Jesus and was killed on a Friday. Christ's crucification day is also considered to be a Friday by many historians.
Interesting Facts About Friday the 13th
* The fear of Friday the 13th has been called friggatriskaidekaphobia (Frigga being the name of the Norse goddess for whom "Friday" is named and triskaidekaphobia meaning fear of the number thirteen). (From Wikipedia)
* In 2012, three unlucky Fridays are 13 weeks apart: January 13, April 13, July 13. Eek! (hasn't happened since 1984)
* Hotels and hospitals often skip floor 13, and some airports don't have a gate 13.
* President Franklin D. Roosevelt is said to have avoided travel on the 13th day of any month, and would never host 13 guests at a meal. (Source)
* Fidel Castro was born on Friday, August 13, 1926.
* Friday the 13th superstitions cost businesses between $800 and $900 billion. People choose not to buy houses, fly, or buy stocks (to name a few things) on this day. (Source)
I found several sites that listed unfortunate events that occurred on Friday the 13ths throughout history. This seemed particularly silly to me... take any day, and you could easily find bad things that happened. Research all Tuesday the 18ths, for example.
Are you wary of Friday the 13th?