Friday the 13th is a date considered to be bad luck in western superstition.
The number 13 also means unlucky, and Friday was a unlucky day. The number 12 means completeness, 12hrs of the clock, 12 months of the year. Even Jesus had 12 disciples, also there was a myth about having 13 people on the dining table will lead one of them to death. Even till this day alot of our buildings skips 13th floor.
Friday also was a unlucky day since the 14th century. Alot of people believed Friday was a unlucky day to start anything new, or even go on journeys. Also, Black Friday has been associated with stock market crashes and other disasters since the 1800s.
We also cant leave out the famous/classic 1980 film "Friday the 13th",
produced by Sean S. Cunningham and written by Victor Miller. Which went on to become one of the most-profitable slasher films in cinema history.
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Its either you believe it or not. Well Jason, its time to go put on your mask, and party hard!
http://en.wikipedia.org/wiki/Friday_the_13th - for more information.