In late 20th century there was a computer virus called 'friday the 13th'.
Which probably transmitted via mails on every 13th day of a month only if it was a friday. (you see, it was programmed that way).
I guess, from there it all started.
Also, 13 is a illuminati number they say, if you believe in such stuff.
If you look at the logo of Baskin Robbins carefully, you will find '13', proving that its in the hands of illuminati.
Dont get into all this, they are all myths also, the friday the 13th virus is no more functional.