Hmmm. still six months away, but anyway, good enough to start collecting idea's. First of all good luck for the remainder of the year on the study!
Self I have not really given a celebration or party when I finished my studies, just kept it in the family. I don't know about you Zeroki, for how you look at it.
I can suggest to keep it close, invite a few friends over at your house? (if parents are okay with that.)
I can think also on going with them to to a sort of attraction of lasergame / bowling / Go kart and having dinner there? Depends on on how big your budget is, you want to spend on it. Choose something you like and they too. December is the month of cold and snow, so outdoor activities can be scrapped.
Ok, that where my ideas and ran out of them. Anyhow i wish you the best on collecting them and finding out what to do on that day.