Sforno on Deuteronomy 16:11/ספורנו על דברים ט״ז:י״א

1והגר והיתום והאלמנה, they shall rejoice on the festival of cutting (the wheat). Their rejoicing will take the form of their collecting the portions of this harvest set aside for them by the Torah, i.e. the פאה, corner of the field with standing grain; the gleanings, what the farmer had overlooked, etc. These have been enumerated in connection with the festival in Leviticus 23,22.