Pentecost is the celebration of the person of the Holy Spirit coming upon the Apostles, Mary, and the first followers of Jesus, who were gathered together in the Upper Room.
A “strong, driving” wind filled the room where they were gathered, and tongues of fire came to rest on their heads, allowing them to speak in different languages so that they could understand each other. It was such a strange phenomenon that some people thought the Christians were just drunk - but Peter pointed out that it was only the morning, and that the phenomenon was caused by the Holy Spirit.
The Holy Spirit also gave the apostles the other gifts and fruits necessary to fulfill the great commission - to go out and preach the Gospel to all nations. It fulfills the New Testament promise from Christ (Luke 24:46-49) that the Apostles would be “clothed with power” before they would be sent out to spread the Gospel.
Where’s that in the bible?
The main event of Pentecost (the strong driving wind and tongues of fire) takes place in Acts 2:13, though the events immediately following (Peter’s homily, the baptism of thousands) continue through verse 41.
Happy Birthday, Church
It was right after Pentecost that Peter, inspired by the Holy Spirit, preached his first homily to Jews and other non-believers, in which he opened the scriptures of the Old Testament, showing how the prophet Joel prophesied events and the coming of the Holy Spirit at Pentecost.
He also told the people that the Jesus they crucified is the Lord and was raised from the dead, which “cut them to the heart.” When they asked what they should do, Peter exhorted them to repent of their sins and to be baptised. According to the account in Acts, about 3,000 people were baptised following Peter’s sermon.
For this reason, Pentecost is considered the birthday of the Church - Peter, the first Pope, preaches for the first time and converts thousands of new believers. The apostles and believers, for the first time, were united by a common language, and a common zeal and purpose to go and preach the Gospel.