When I get to trying to track down useless knowledge, I sometimes don't know where to stop. All you need to know about triskaidekaphobia

It's not clear to me if the unlucky 13th at the Last Supper was Jesus or Judas, I've found references to both. The most complete information was in a Smithsonian article I found. Here's the best part.

"Triskaidekaphobia can strike when you're most expecting it. (fear of the number 13), " by Paul Hoffman in Smithsonian, Feb 1987 v17 p122(9)
Triskaidekaphobes, the original Greek for those suffering from the morbid fear of the number 13, can muster ample historical evidence in support of their affliction. After all, Jesus was 13th at table and look what happened to Him. The belief that 13 people cannot dine together at the same table without fatal consequences actually goes back even farther than the Last Supper. Some popular accounts have it that in Norse mythology, the demonic god Loki crashed a dinner party of benevolent deities, making their number 13 and thus causing the death of Balder, the god of light.

The idea that numbers are not mere instruments of enumeration but are sacred, perfect, friendly, lucky or evil goes back to antiquity. In the sixth century B.C., Pythagoras … made a religion out of numbers. In numerology, the number 12 has always represented completeness, as in the 12 months of the year, the 12 signs of the zodiac, the 12 hours of the day, the 12 gods of Olympus, the 12 labors of Hercules, the 12 tribes of Israel, the 12 Apostles of Jesus, the 12 days of Christmas and so on. Since 13 exceeds 12 by only one, the number lies just beyond completeness and, hence, is restless to the point of being evil.

Friday the 13th is doubly potent because it draws not only on the evil power of the number 13 but on Friday's reputation as a day of bad luck. Since Christ was crucified on a Friday, the day can be seen as a time for endings, not beginnings. It has even been claimed through guilt by association that Eve gave Adam the apple on a Friday. Friday is no day for getting married, embarking on a journey, moving into a new house, starting a new job, weaning a child, cutting your nails or turning over a mattress. Moreover, until the end of the 19th century, Friday was known as Hangman's Day and was reserved for executions.

... as recently as 1978 it was estimated that triskaidekaphobia costs America a billion dollars a year in absenteeism, train and plane cancellations and reduced commerce on the 13th of the month.