데바림 (신명기) 16:15

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).

שִׁבְעַ֣ת יָמִ֗ים תָּחֹג֙ לַיהֹוָ֣ה אֱלֹהֶ֔יךָ בַּמָּק֖וֹם אֲשֶׁר־יִבְחַ֣ר יְהֹוָ֑ה כִּ֣י יְבָרֶכְךָ֞ יְהֹוָ֣ה אֱלֹהֶ֗יךָ בְּכֹ֤ל תְּבוּאָֽתְךָ֙ וּבְכֹל֙ מַעֲשֵׂ֣ה יָדֶ֔יךָ וְהָיִ֖יתָ אַ֥ךְ שָׂמֵֽחַ׃

Seven days shalt thou keep a solemn feast to the Lord thy God in the place which the Lord shall choose: because the Lord thy God shall bless thee in all thy produce, and in all the work of thy hands, therefore thou shalt surely rejoice.