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והיית אך שמח. לְפִי פְשׁוּטוֹ אֵין זֶה לְשׁוֹן צִוּוּי אֶלָּא לְשׁוֹן הַבְטָחָה, וּלְפִי תַלְמוּדוֹ לָמְדוּ מִכָּאן לְרַבּוֹת לֵילֵי יוֹם טוֹב הָאַחֲרוֹן לְשִׂמְחָה (שם מ"ח): והיית אך שמח ONLY BE REJOICED — According to its plain sense this is not the expression of a command but expresses an assurance: “thou will be rejoicing”. But according to the Halachic interpretation they (the Rabbis) derived from here that the night before the last day of the festival (that preceding the eighth day) is to be included in the obligation of rejoicing (Sukkah 48a; cf. Sifrei Devarim 142:4).