Rashi on Deuteronomy 16:13/רש"י על דברים ט״ז:י״ג

1באספך [THOU SHALT KEEP THE FESTIVAL OF TABERNACLES …] AFTER THAT THOU HAST GATHERED IN THE PRODUCE — i.e. at the usual harvest time, when thou bringest into the house the summer fruits. Another explanation is: באספך מגרנך ומיקבך teaches that one should cover the Succah only with the פסולת (lit., the chips, — that which falls off) of the barn and the wine-press [i.e. with vegetable matter] (Rosh Hashanah 13a; Sukkah 12a).