For Christians Pentecost is a day commemorating the descent of the Holy Spirit upon the Apostles and other followers of Jesus Christ (120 in total), as described in Acts 2. It is celebrated fifty days (i.e. 49 days with the first day counted, seven weeks) after Easter Sunday, hence its name.
Pentecost is sometimes referred to by Christians as the “Birthday of the Church”. This is because the event in Acts 2 marks the first large-scale spreading of the Gospel after Jesus’ ascension. After receiving the Holy Spirit, Peter preaches to the crowd in Jerusalem, gathered from around the world to celebrate Pentecost, and some 3,000 people are saved and baptized.
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.