Leviticus 22-24…Unnacceptable sacrifices and the appointed festivals.
Passover was not the only spring festival celebrated under the covenant, for the Israelites also commemorated the Feast of Firstfruits and the Feast of Weeks, or Pentecost. The Feast of Firstfruits actually took place during the week-long Passover celebration (Leviticus 23), on the first day after the Sabbath that occurred in the midst of the week. Pentecost occurred fifty days after that Sabbath and marked the culmination of what started at the Feast of Firstfruits.
As its name indicates, the Feast of Firstfruits marked thanksgiving to God for the firstfruits of the harvest — in this case, the grain and cereal harvested in the spring in ancient Palestine. At this festival, the Israelites offered the very first sheaf of the harvest and were not allowed to eat anything from the crop until they gave its initial portion to the Lord. This required a great deal of faith on the part of the Israelites, as they would be giving the offering of firstfruits at a time when not much was ready to be harvested. They had to trust God that He would indeed provide the fullness of grain that had yet to come forth, something that from a human perspective was far from certain given the people’s utter dependence on the right amount of rainfall and so forth to give the best crop.