In the New Testament, the day that the Holy Spirit descended upon the disciples of Jesus is referred to as the Pentecost.
Pentecost is celebrated on the seventh Sunday after Easter, commemorates the descent of the Holy Spirit upon the Apostles and other followers of Jesus Christ while they were in Jerusalem celebrating the Feast of Wheats, as described in the Acts of the Apostles (Acts 2:1–31).
Pentecost is the Greek name for Shavuot, the spring harvest festival of the Israelites, which was going on when the Holy Spirit came.
Pentecost is a Christian holy day that celebrates the coming of the Holy Spirit 40 days after Easter.
Some Christian denominations consider it the birthday of the Christian church and celebrate it as such. Originally, Pentecost was a Jewish holiday held 50 days after Passover.
The purpose of Pentecost then is the introduction to God doing something He has never done before, that of filling the faithful with His Spirit. The purpose is that we become witness to His character, presence, power, and authority.