The Jewish feast of Pentecost, in Hebrew Shavuot , the Feast of Weeks, one of the three pilgrimage festivals, arose as the celebration of the closing of the spring grain harvest, which formally began 50 days earlier at Passover; there are numerous references to it in the Bible, e.g. Exodus 34:22 and Leviticus 23:15. From Rabbinic times, the festival commemorates the giving of the law to Moses at Mt. Sinai.
On the Pentecost after the resurrection of Jesus (50 days from the Passover in which He was crucified), the Holy Spirit, according to the Book of Acts chapter 2, descended on the disciples in the form of tongues of fire accompanied by the sound of a rush of wind, and gave them the power of speaking in such a way that people of different languages could understand them.
In liturgical Christianity, Pentecost is an annual commemoration of this event, and it is solemnly observed as the birthday of the church and the feast of the Holy Spirit.