Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 668…
“ We do not refer to the eighth day as a "festival" because there is nowhere [in the Torah] that it is called a festival. Rather, we say "the eighth day of Atzeret." Outside of Israel, they eat in the sukkah at night and during the day because it might be the seventh day. They do not bless on this sitting. ”
