Pentecost commemorates the descent of the Holy Spirit upon the Apostles and other followers of Jesus as described in the New Testament Acts 2:1-31. For this reason, Pentecost is sometimes described as the "Birthday of the Church".
"When the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them. Now there were staying in Jerusalem God-fearing Jews from every nation under heaven. When they heard this sound, a crowd came together in bewilderment, because each one heard them speaking in his own language". Acts 2:1-6