From the introduction:
Now you’ve probably wondered where holidays come from.
If you haven’t I’d say it’s time you begun.
This is a world where human holidays exist as towns in another dimension. Each of these towns is full of everything most commonly associated with that holiday, Easter is full of chocolate eggs and rabbits, Christmas is full of elves making presents, etc.
You are a resident of Halloween Town - a town full of monsters, spookiness and good natured mischief.
You arrive shortly before the Halloween at the start of the movie, just as the last preparations are being done to make this the scariest one yet. Soon enough Jack Skellington, the king of the pumpkin patch, will find his way into Christmas Town and attempt to claim Christmas for his own.