שִׁבְעַ֣ת יָמִ֗ים תָּחֹג֙ לַיהוָ֣ה אֱלֹהֶ֔יךָ בַּמָּק֖וֹם אֲשֶׁר־יִבְחַ֣ר יְהוָ֑ה כִּ֣י יְבָרֶכְךָ֞ יְהוָ֣ה אֱלֹהֶ֗יךָ בְּכֹ֤ל תְּבוּאָֽתְךָ֙ וּבְכֹל֙ מַעֲשֵׂ֣ה יָדֶ֔יךָ וְהָיִ֖יתָ אַ֥ךְ שָׂמֵֽחַ׃
Sieben Tage sollst du dem HERRN, deinem Gott, ein Fest feiern an dem Ort, den der HERR wählen wird; denn der HERR, dein Gott, wird dich segnen in all deiner Zunahme und in all dem Werk deiner Hände, und du wirst insgesamt fröhlich sein.
Rashi on Deuteronomy
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).
Sforno on Deuteronomy
כי יברכך, so that you will have a great deal of gathering in to do, and as a result, the poor will also find much to collect.
Tur HaArokh
שבעת ימים תחוג לה' אלוקיך, For seven consecutive days you shall celebrate to the Lord, your G’d;” this imperative is directed at the entire community, or Moses exhorts every individual to offer up his burnt offerings and his peace offerings.