As the heading says:
How do you refer to the day/half day of fun activities that schools usually have at the end of term?
I'm not sure what it's like in English speaking countries, but I guess you have something similar as we in my country do – on the last day of school before the Christmas break or summer break, kids gather in their classrooms and have something nice to eat, perhaps watch a film etc, and then the whole school (or less, depending on how big the school is) gather in the auditorium together with friends and family, and the different classes perform in different ways, and so on. In my own language we call this day "finish", so "Christmas finish" and "summer finish", but I'm pretty sure that's not what it's called in English ;)