1. There's a real disorder centered around it.
Friggatiskaidekaphobia is the irrational fear of friday the 13th. People with this disorder tend to believe that something extraordinarily evil must happen on that day.
2. It's Proven (To some extent)
It was on a Friday June 13th. Just after reading an article on the mystery behind Friday the 13th, I saw a headline reading "Singer Kefee Confirmed Dead". I thought it was a hoax at first but it later proved to be true.
Tupac died on friday September 13th and the list goes on....
3. It's Bad News for The Government
The aviation industry is as important as any in developed countries. Friday the 13th costs the US economy 800million dollars each year because people refuse to travel by air.
4. Its Mostly Based on Folklore.
There's little concrete proof of friday the 13th being exceptionally evil and unlucky. Most of the "proof" is based on folklore from mythology.
5. It's Proof of "Group Mentality"
Like number four says, there's no real proof yet people fear the date. Doesn't that make you wonder how many things you are doing, not because you want to but because its based on something no one is sure how it even began. Like greeting people, waking up in the morning and going to bed at night, laws etc.

Also heard about sit number 13 in the aeroplane is not allow, unit 13, ward 13 and room number 13 in the hospital or in a care home is not also allow.. I don't know mystery behind this number.