Deuteronomy 16:9,10

Thou beginnest to put the sickle to the corn To reap the first-fruits of the barley-harvest, the wave sheaf in particular, which was offered to the Lord on the sixteenth day of that month. Thou shalt keep the feast of weeks So called, because it was seven weeks after the bringing the sheaf at the passover, that is, fifty days, whence it was termed pentecost: see on Leviticus 23:16. It was also called the feast of first- fruits, Numbers 28:26. With a tribute of free-will-offering Over and besides the sacrifice for the day, and the two loaves and sacrifices with them, Numbers 28:27; Leviticus 23:17. God here directs that they should make some voluntary oblation.