One of the traditions that baffled them is that the person celebrating their birthday is expected to arrange for a cake and/or little gifts for their friends. We call it 'uitdelen', which roughly translates to 'distributing'.
It's especially popular in primary school over here. You even visit all the classrooms and give the teachers a little snack to celebrate your birthday with them.
I'm not 100% sure if it's a Dutch-only thing, but I'll stick with this little tradition. Use discount code UITDELEN to get a 10% discount on all yarn sets and celebrate my (Aiden's) birthday!