A fear of Friday the 13th is called paraskevidekatriaphobia. The term paraskevidekatriaphobia is based on the Greek words:
paraskevi - Friday     dekatria - thirteen     phobia - indicates fear

Unlucky Day

The belief that Friday 13th is an unlucky day is one of the most widely known superstitions in Britain today.

The superstition surrounding Friday the 13th is a combination of two separate fears:

    the fear of the number 13 (called triskaidekaphobia)
    the fear of Fridays

Unlucky 13

It is unlucky to have thirteen at a table or in a company. The number 13, in the Christian faith, is the number of people at the Last Supper, with the 13th guest at the table being the traitor, Judas.


Why are people superstitious of the number 13?

    A witches coven consists of 13 members.
    Tarot Card number 13 is the Death Card, depicting the Grim Reaper
    Hotels rarely have a room number 13
    There are 13 steps leading to the gallows.
    13 knots in a hangman's noose.
    13 feet which the guillotine blade falls.
    The driver of Princess Diana hit pillar 13 at Place de l'Alma, Paris, France which unfortunately resulted in her death.

Fridays

    Fridays have been considered an unlucky day since the medieval times.
    It is generally considered bad luck to start anything on a Friday - a new journey, job, marriage, business project, giving birth, moving house
    It is unlucky to cut fingernails and hair on a Friday.
    In Britain, Friday was the conventional day for public hangings, and there were supposedly 13 steps leading up to the noose!

Friday is consider an unlucky day in some religions

Muslims tout Friday as the day Allah created Adam.
Adam and Eve ate the forbidden fruit on a Friday and later died on a Friday.
Christians consider Friday as the day on which Christ was crucified by the Romans.    
                  
 Valami hasonlóról olashatsz magyarul itt: http://www.szeretlekmagyarorszag.hu/pentek-13/