The Feast of Weeks, also known as Harvest (Exodus 23:16), Shavuot (Hebrew), the Day of Firstfruits (Numbers 28:26), or Pentecost, was a festival of joy and thanksgiving celebrating the completion of the harvest season. It was the second major feast in which all able-bodied Jewish males were required to attend (the other two being Passover and the Feast of Tabernacles). It was celebrated as a sabbath with rest from ordinary labors and the calling of a holy convocation (Leviticus 23:21; Numbers 28:26).
Essentially a harvest celebration, the term weeks was used to describe the time period from the grain harvest to the barley harvest and finally to the wheat harvest. It is called the Feast of Weeks because God specifically told the sons of Jacob that they were to count seven sevens of weeks (seven complete weeks) from Firstfruits (Leviticus 23:15; Deuteronomy 16:9), and then on the "morrow" this fourth feast was to be observed (Lev. 23:16). Seven sevens of weeks are forty-nine days. Add one additional day ("on the morrow"), and it brings the total number of days to fifty. This fourth feast was to occur precisely fifty days after Firstfruits (Christ's resurrection). Therefore, the feast was given the name "Pentecost" (Acts 2:1) which means "fifty."