Paraskevidekatriaphobia, the fear of Friday the 13th is where people avoid travelling, marrying, or even working that day. As for the latter, you have to be pretty irrational to not see that not showing up for work for suspersitious reasons is is much more likely to have “unlucky” consequences.

Judas was the 13th guest to the last supper, who betrayed Jesus. The Norse god Loki was the 13th attendant at a dinner party, creating chaos for the 12 other gods in attendance.

13 is considered lucky in parts of south and southeast Asia, such as India and Thailand. In both cultures, they primarily use the base-10 numbering system as we do.

In Christianity, 13 often seems to be a play on the duodecimal (base-12) system. There are some mathematical qualities of 12, in that it has a lot of divisors: it can be divided into sixths, quarters, thirds, and halves, and still have integer values. Adding 1 to 12 makes 13, which is prime, and not much fun for integer division. While that could introduce a kind of chaos, you also get a prime if you subtract 1. So why isn’t 11 unlucky? Seen another way, a prime number can be regarded as lucky in itself and as seen as symbolic of wholeness in some cultures.