Triskaidekaphobia was widespread in Europe by Medieval times and in other places, such as ancient Persia. But, among other places and peoples, such as with Jews and Sikhs, the number 13 is a positive number (perhaps triskaidekaphilia). There is much speculation about how triskaidekaphobia came about--stories about 13 attendees at the Last Supper, etc.
But, what is more interesting, at least to me, is why the number 13 was a sacred number in pre-Christian, and further back in pre-Indo-European, Europe. I attended a lecture of professor Marija Gimbutas, UCLA archeology professor of Indo-European studies and of "Old Europe" (before the Indo-European invasions of the 5th millenium BCE), and twice talked to her on the telephone around 1991 (unbeknownst to me she was dying at the time) and bought one of her expensive books. She claimed, as do others, that a very important part of Old European (pagan) religion (which existed for tens of thousands of years) was based on the number 13, because the lunar calendar has 13 full moons or new moons. Also, there are 13 days of waxing moon before a full moon and then 13 waning days.
Taking this one step further, there is correlation between moon cycles and the human female menstrual cycle. Afterall, the mens in that word refers to moon, referring to 13 female menses in a year. It should, then, not be surprising that Old Europe had a moon goddess.
Long before Christianity, according to Gimbutas, when Indo-Europeans invaded Old Europe, the peaceful, egalitarian, Mother Earth-based religion which had priestesses was supplanted by a warlike, male sky god-based religion. The old religion had to be destroyed (usually the first project of domination); thus, the number 13 was made rather taboo. And, females were relegated to inferior status: no priestesses, no moon goddess, little respect.
It makes a good story, although I'm sure it is controversial among archeologists. I've not heard anything lately about this area of thinking. There is a very interesting book that includes this information called The Chalice and the Blade: Our History, Our Future, by Riane Eisler, first published in 1987, with a new epilogue in 1995. You ardent feminists must read this (but surely you already know of it).