Where do superstitions come from?

[Boreas]

It's said that the thing about 13 comes from the Last Supper (12 Disciples plus Jesus).

The Celts had a deity that took the form of a black cat. The aversion to them may stem from Rome's suppression of the Celtic religion and culture in Western Europe and Britain.

Green? I don't really know. Might be Celtic again, with"the little people". That might be why some consider it good luck and others bad luck.

I think all superstitions come from folklore (duh) and, since folklore is so rich and diverse, there's no one source for superstitious beliefs and some superstitions go back so far, way back into pre-literate cultures, that finding the root is impossible in some cases.

John