Susan had never hung up a stocking . She'd never put a tooth under her pillow in the serious expectation that a dentally inclined fairy would turn up. It wasn't that her parents didn't believe in such things.
They didn't need to believe in them. They know they existed. They just wished they didn't.
It's the night before Hogswatch. And it's too quiet. Where is the big jolly fat man? There are those who believe and those who don't, but either way it's not right to find Death creeping down chimneys and trying to say Ho Ho Ho.
Superstition makes things work in Discworld, and undermining it can have Consequences, particularly on the last night of the year when the time is turning. Susan the gothic governess has got to sort everything out by morning, otherwise there won't be a morning. Ever again...
The 20th Discworld novel is a festive feast of darkness and Death (but with jolly robins and tinsel too). As they say: 'You'd better watch out...'