Paul takes us all the way back to Leviticus 23 and the inauguration of the Feast of Firstfruits, where the very first sheaf of grain in every smallholding, every farm in ancient times, was to be brought to the priest and the priest would wave it in the air. This was before the main harvest; the rest of the harvest was brought in later, and the end of harvest was marked by the Feast of Weeks seven weeks later (Leviticus 23:15-22).