Any unusual event is reason for a party, and in the good old days (hm) it was also tradition to have a party when it was time to throw out the Christmas tree. This of course arose from the original habit of decorating the tree with edible things, like apples and ginger biscuits and sweets.
Old Christmas Card by Jenny Nyström
I'm not sure how popular the tradition of julgransplundring ("plundering of the Christmas tree") still is. Back in my childhood in the 60s we still had them (for children), even though Christmas tree decorations weren't usually of the edible kind any more. But cakes and sweets were served, and there were games and singing and dancing around the tree; and before you left you also got a bag of sweets to take with you.
I very rarely if ever got to have a birthday party of my own as a child (partly perhaps because I shared birthday with my father). But there were a few getting-rid-of-Christmas-parties.
Today I've started taking down the decorations in my flat, but somehow it feels more like a lot of work than a party...! I think I'll wait a couple of days before I take the box down to the storage room... In some mysterious way, just when you think you've caught all the gnomes, you usually disover another one lurking in some obscure corner...