פירוש על דברים 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).

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.

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.