Pentecost

Pentecost (“the fiftieth [day]”) is the ancient Greek name for the Feast of Weeks, a prominent feast in the calendar of ancient Israel celebrating the giving of the Law on Sinai. This feast is still celebrated in Judaism as Shavuot.
For Christians, it is a day commemorating the descent of the Holy Spirit upon the apostles and other followers of Jesus Christ as described in Acts 2.
Because of their belief that the baptism in the Holy Spirit is a unique experience which Christians can still have today, the Pentecostal movement derives its name from this New Testament event.