Leviticus 23:21

An holy convocation, a sabbath or day of rest, called pentecost, which was instituted, partly in remembrance of the consummation of their deliverance out of Egypt, by bringing them thence to the mount of God, or Sinai, as God had promised, and of that admirable blessing of giving the law to them at that time, and forming them into a commonwealth under his own immediate government; and partly in gratitude for the further progress of their harvest, as in the passover they offered a thank-offering to God for the beginning of their harvest.