The ancient vampires here in the lair are saying that Friday The 13th reportedly evolved from Norse mythology. You already know that Friday is named after the beautiful love-Goddess Frigga, right? Well, when Christianity took over, belief in Frigga was not merely ridiculed but the church banished Frigga as a "witch" and insisted she convened with 11 other witches and the devil — a gathering of 13.
Hence we observe the day and number affiliated with Frigga as "evil." Beyond that, we have a medical diagnosis called "friggatriskaidekaphobia," which describes people who suffer "Friday the 13th phobias." (Notice how Frigga's name is inside that diagnosis). Some other tales involving the Knights Templar being murdered on Friday the 13th also surround this mini-Halloween-like holiday.
Be afraid. Be VERY afraid. Vlah-haha. (I am joking of course. You're completely safe near me. See my glowing red eyes?)
~ Vamchoir (aka Ravena Doomlah)