John Calvin on Deuteronomy 16:9-10

9. Seven weeks shalt thou number. It must be observed that the Passover fell in a part of the year when the harvests were beginning to ripen; and consequently the first-fruits, of which I treated under the First Commandment, were then offered. Seven weeks afterwards they celebrated another feast-day, which was called Pentecost, i.e., the fiftieth, by the Greeks. There was just this number of days between the departure of the people and the publication of the Law. Another offering of first-fruits was then made, in which each one, according to his ability, and in proportion to the produce of the year, consecrated a gift to God of the harvested fruits. In order that they might be more ready and cheerful in their liberality, God’s blessing is set before them, as if Moses had commanded the people to testify their gratitude; since whatever springs from the earth, is the mere bounty of God Himself.