The word for those who are afraid of Friday the Thirteenth is paraskevidekatriaphobia. It's a not-uncommon fear.
But is it all just superstition or is there some valid basis behind this fear?
There are many articles and experts who can help you draw conclusions regarding the origin of such concerns, but the history is at best sketchy. Is it, then, part of some retained ancient wisdom that is able to highlight the days on which evil could triumph? Or is it rather the foolish man's misunderstanding of various unrelated stories fashioned into popular myth?
The biggest problems with Friday the 13th lie with two fundamental issues.
Firstly, the number of fingers that we have.
And secondly, the absence of better telescopes.
Let me explain.
Firstly, the fingers.
Human kind has ten fingers. Because of this we work with what is called a base-10 numbering system. In other words, once we get to nine, we then start afresh from zero with a one (1) before it all. Thirteen, therefore is represented as 10 plus 3 -> 13. This counting system is quite significant. Computers use binary (0, 1, 10, 11, 100 etc) for example.
But if we had only 8 fingers, then once we got to seven we would then introduce that one (1) and restart from zero again. Consider the following table which shows the numbers from one to thirteen in both base-10 as well as base-8:
As you see, with base-8 we would get to '13' quickly as the written equivalent of eleven. Similarly, thirteen would be written as '15'.
Of course, in base-2 i.e. binary, thirteen must be written as 1101 and there is in fact no meaning in the word "13" since only ones (1) and zeroes (0) are used.
Now that throws numerology and other systems off somewhat.
So if we had been born with 8 fingers (or in fact 9 or 11 etc) then our understanding of 13 would be massively different.
Then, there is the problem of the absence of better telescopes.
The main reason that the seven day week was chosen was in part (at least) due to the significance of the number seven, and specifically because there were seven known planets.
Sun, Mercury, Mars, Venus, Moon, Jupiter, Saturn
Poor old Pluto and Neptune had not been discovered!
If they had then we might well have had 9 day weeks which, apart from giving us either a very long working week (or a very long weekend!) would also have affected the days on which our current Friday the Thirteenth occurs. In other words, because the weeks would be 9 days long, the day which we currently associate with Friday the Thirteenth would probably be a Monday instead. Or a Saturday. Or a Gooday or a Gnomeday..... or whatever they chose to call the extra two days.
The practical upshot of all this is that the whole association that we have with a day being a Friday and the 13th of a month is completely arbitrary and totally unrelated to fate or evil or whatever.
So therefore, there can be no sensible way in which we can assume that any Friday the Thirteenth is unlucky!