Why is Friday the Thirteenth Thought to be Unlucky?

There's a lot of speculation about that age-old superstition, but very few plausible answers. Because Jesus was crucified on a Friday seems to top the list, but I don't buy it. For one thing, the day Jesus died on the cross is called Good Friday, not Bad Luck Friday--and, as far as I can see, has nothing whatsoever to do with the number thirteen. Which begs the question: why is the number thirteen considered unlucky? Again, some sources tell us it's because there were thirteen present at the Last Supper (does that include the Holy Spirit?--or did he show up later?). There's also speculation about hosts of other tragic historical events in which thirteen people were present, but those seem just a little contrived in my opinion. I've long held that the reason thirteen is considered unlucky derives from the tarot. In the Major Arcana, the thirteenth card is the one pictured herein. Modern tarot readers will tell you (and I don't disagree) that the card signifies major change or spiritual transformation. But did it always mean that? And why do we worry about black cats crossing our path on Friday the Thirteenth? Black cats were believed to be the alter egos and familiars of witches. So, does reason follow that the superstition is about Jesus . . . or does it perhaps have more ancient roots? Inquiring minds want to know . . .