Matthew Henry on Leviticus 23:15-22

The feast of Weeks was held in remembrance of the giving of the law, fifty days after the departure from Egypt; and looked forward to the outpouring of the Holy Ghost, fifty days after Christ our Passover was sacrificed for us. On that day the apostles presented the first-fruits of the Christian church to God. To the institution of the feast of Pentecost, is added a repetition of that law, by which they were required to leave the gleanings of their fields. Those who are truly sensible of the mercy they received from God, will show mercy to the poor without grudging.