Absolutely this is a non-Jewish superstition, based on a Jewish occurrence. Judas was the 13 person present at the so-called last supper which took place, so the non-Jews say on what is now referred to as Good Friday. As Judas sold his mentor out to the Romans for some pieces of silver, 13 has since then been considered unlucky by non-Jews. We on the other hand, believe 13 to be of great significance. A boy becomes a bar-mitzvah at age 13. There are 13 Articles of Faith, 13 Middos of Hashem (?). In the moment now, I cannot think of other examples, but I am certain there are. For a Yid, 13 is a good number.
BTW, I am not sure “holiday” is how I would describe their irrational concerns about the day.