The Discworld is very much like our own - if our own were to consist of a flat planet balanced on the back of four elephants which stand on the back of a giant turtle, that is . . . OH, THERE HAS TO BE SOMETHING IN THE STOCKING THAT MAKES A NOISE, said Death, OTHERWISE WHAT IS 4:30 A.M. FOR?
Superstition makes things work in the Discworld and undermining it can have Consequences.
It's just not right to find Death creeping down chimneys and trying to say Ho Ho Ho . . .
It's the last night of the year, the time is turning, and if Susan, Gothic governess and Death's granddaughter (sort of), doesn't sort everything out by morning, there won't be a morning. Ever again . . .