What’s so unlucky about the number 13?
Many cultures have found 13 unlucky, but it is not exactly clear why. One theory is that there are 12.4 lunar cycles per year, so people might have experienced it as 12 full months + reminder, which made 13 (the inferior "reminder") look worse than the other numbers.
Many believe it is UNLUCKY because Judas Iscariot double-crossed Jesus on the date, Friday 13 October, 1307. So they believe if someone gets #13, it betrayed him/her.
It's still a conundrum.