Friday the 13th

Back in high school, I had a history teacher who was afraid of the number 13, quite literally. He created little hand written revision books for each of us with all of the course content in them. Noticeably, he skipped the number 13 when writing the page numbers on. It would go p.11,12, 12B,14, and so on. On the few Friday the 13th's we had during school, he was mysteriously absent. It's fair to say, there's no chance we'd be learning about the Battle of Gammelsdorf of 1313.