In the books, Thirteen is a good number.
There are these shapeshifting predators that move through a secret thirteenth hour that happens at midnight. They dislike anything artificial and new. Not just solid things like steel or technology, but also concepts like numbers.
They hate thirteen particularly. If you say a thirteen letter word (Languages are also a new concept), it actually hurts them a little. A person who knows what they are doing can make weapons of a sort, taking things with favorable dates it was made or nice materials, and give it a thirteen letter name. This makes it burn darklings just by touching them.
As for why anyone would be in there, normally people freeze during the midnight hour. For them, it passes in less than a second. But someone who was born at that exact moment.. they are naturally a part of it.
Most of them have odd talents. Like Acrobats- who live in low gravity during that hour, Polymaths- for whom numbers and mathematics are like breathing, Mindcasters- who can read minds and even control them at times.. and some others..
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather