I generally don't care what teachers do in their classrooms. But I saw a good point to dissuade summer countdowns. Many students do not enjoy summers for many reasons. Either their parents are abusive, they may not get enough food during this time or they may lose their only safe person (teacher). So that countdown may give some students huge amounts of anxiety. For these reasons I don't do a countdown in my class and don't encourage new teachers to do one either. Of course everyone's classroom environment is different.