Triskaidekaphobia (fear of the number thirteen) isn't only related to Fridays: in many parts of the world, Tuesday is the feared "13th".
No-one seems to know how the two elements came together or what were their individual origins. By the Middle Ages, Friday was already considered an unlucky day, perhaps because Christ was crucified on a Friday. However that is at odds with references to Good Friday being the only Friday which is not unlucky.
Superstition about the number 13 arrived much later, not until the 18th century and no-one seems certain where it came from although some say it is because Judas was the 13th disciple.
Whatever the reason, fear of the number 13 became more widespread during the 19th century, but it was only about an hundred years ago that the two superstitions were combined.
Foolish maybe, but Wikipedia notes it is estimated that US business loses $800-$900 million dollars each Friday 13th because of people staying at home, unwilling to travel.
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