If you suffer from either of these phobias, you might want to keep a low profile today, and be extra cautious in everything you do. Avoid walking under ladders, and give any black cats a wide berth, because today of course, is Friday the 13th! The two phobias above are, respectively, a fear of the number 13, and a fear of Friday the 13th specifically.
Though the superstition is on the wane, you can still find people who consider today unluckier than the average, and a breath a sigh of relief when it’s over (though if you’re a believer in the day, unfortunately there’ll be another one this year). But why is Friday the 13th considered unlucky? There are many different theories about why the number 13 specifically is considered unlucky. One of Christian origin is that the number is unlucky because Judas was the 13th person to arrive at the Last Supper.
Other suggestions of an older origin for the superstition concern lunar cycles. In ancient times, a lunar calendar was in use and the months aligned with the waxing and waning of the moon, A year with 13 full moons, and thus 13 months, was considered a pain for priests who had to rearrange religious celebrations to accommodate this, and this may explain how the number became unlucky. Relatedly, some consider that it might have been caused by the fact that the number 13 represented femininity in ancient cultures, as there were 13 lunar (or menstrual) cycles per year (13 x 28 = 364). Making the number 13 unlucky might then have been a way to maintain patriarchy, or suppress lunar cults.
The superstition of the number 13 is probably a mix of all these things: the number 12 is an important number in so many cultures and religions (12 apostles, 12 days of Christmas, 12 Tribes of Israel, 12 Jyotirlingas in Hindu Shaivism), and in measurement of time and distance (12 months in a year, 12 inches in a foot). 13 could then be seen as a step beyond normality and order, especially because, unlike 12, it’s an odd number. It’s not surprising then that many hotels have neither a Room 13, or a 13th floor. Even in Ireland, the car licence-plate system (which previously featured the last two digits at the beginning, e.g. 12-G-44632 for a car bought in 2012) was changed in 2013. Since then, registration numbers have featured a 1 or 2 after the two digits to indicate in which half of the year it was bought (e.g. 131 for a car bought between January and June 2013), partly to allay superstitions about a registration number with 13 being unlucky, and sales therefore being down.
But why Friday the 13th? The most common belief is that it has its origin in order for the arrest of the Knights Templar by King Phillip IV on Friday 13 October 1307. Which was certainly an unlucky day for them, but despite this apparent origin, the first mention of Friday the 13th as a particularly unlucky day didn’t occur until 1908, and even then the article which mentioned it might have been more concerned with the fact that a senator was trying to introduce 13 bills. As the linked article notes, Friday had long been considered a bad day for starting new adventures (probably for the practical reason that most people are winding down and some places would have been closed at the weekend), and it just took some time for someone to put this unlucky day and unlucky number together.
So while I don’t believe in the superstition, I can see how it would come about. And wherever you are, there’s a good chance that there’s lots of snow about, so if you’re travelling, be careful out there.