Sforno on Leviticus 23:39/ספורנו על ויקרא כ״ג:ל״ט

1אך בחמשה עשר יום, after the Torah had discussed the subject of the festivals in general terms, i.e. their common denominator being that these days are called מקראי קודש on which fire offerings on the altar are being offered on the altar in the presence of the Lord, the Torah continues 'אך בחמשה עשר יום וגו, by showing that the festival of Sukkot is different from all the other festivals: 1) the eighth day of that festival is called as we know from וביום השמיני שבתון, that the eighth day of that sequence of days is to be regarded as a kind of Sabbath. Such a concept does not exist as a special, day in the “week,” nor on the festival of matzot, neither in connection with months or years. In respect of all of these only the number seven has a connotation of holiness, not the number eight. 2) This is the only festival in which the Jew is required to change his home, move out of his regular home, as stated clearly in verse 42. 3) it requires that every Jew take four plants of specific categories and wave them in the 6 directions on earth. (compare verse 40)