שבעת ימים תחג לה׳ אלקיך ... והיית אך שמח
Rashi comments that v’hayisa ach sameach is not a command, but rather, a promise that we will fulfill the mitzvos of celebrating the festival amid joy. Chazal (Pesachim 71A) derive from here that the joy inherent in our celebration of the festival applies to the last night as well. The commentators question how ach, which in Hebrew is used to exclude, is used in this context to include the last night of the Yom Tov for joyful celebration. Throughout Shas, however, the word is used to exclude. In a homily rendered following the Holocaust which decimated European Jewry, HoRav Shmuel…