The origin of Friday the Thirteenth, as told by Damond Benningfield, comes from Norse mythology.[1] He writes:

First comes the fear of the number 13. According to one tale, 12 Norse gods held a banquet at Valhalla. A thirteenth god — Loki, the spirit of evil — tried to attend…

If you believe that the Norse have little impact on modern culture other than to inspire Marvel comic book movies (and an Emmy nominated television series on the History channel, Vikings), keep in mind the days of the week are named after Norse gods (Tiu’s Day, Woden’s Day, Thor’s day and Freya’s day). Here’s what Benningfield writes about Freya’s day:

Mythology says that when Norse tribes converted to Christianity, Freya was called a witch and banished to a mountaintop. There, every Friday, she hosted a coven of 11 other witches plus the devil — 13 in all — to plot vengeance against her former believers.

So, TGIF. And while you are in enjoying the day, give a listen to Óláfs Saga Tryggvasonar in the Viking language.[2]

NOTES:
[1] You can read the details at Star Date, “Friday the Thirteenth”: http://stardate.org/radio/program/friday-thirteenth
[2] Learn from the Viking Language series: https://soundcloud.com/viking-language/lesson-3-5-l-fs-saga