Acts 2: 1 - 4 (NIV): 1 When the day of Pentecost came, they were all together in one place. 2 Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. 3 They saw what seemed to be tongues of fire that separated and came to rest on each of them. 4 All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
Pentacost was also called "The Feast of Weeks". It was fifty days after Passover and was a Jewish celebration holiday. It celebrated the first harvest of the year. It was one of three major Jewish Holidays that required Jewish men to travel to Jerusalem to celebrate.
It is a major event in the life of Christian believers because it was the day the Holy Spirit came in power and the church age began. Jesus appeared on and off for 40 days after his resurrection, then ascended back to heaven. Ten days later, the Holy Spirit came during Pentecost. After the Holy Spirit came upon the believers, Peter stood and preached to the international crowd that had gathered for the holiday. Three thousand believed in Jesus and the age of the church began. We are still in the church age and waiting for the next great event - the second-coming of Christ.
Pentecost is a celebration of harvest, and the harvest of people's souls began on that day and continues to this one. Jesus had promised that when he went back to heaven, he would send the Holy Spirit to live in us until he comes again (John 16:7). The Holy Spirit appeared as "tongues of fire" which was symbolic of speaking God's purifying truth. A fire begins small and spreads. In the same way, the Gospel began with the disciples and continues to spread around the world. The disciples spoke in other tongues as a demonstration of God's power, and that power is still alive and active today as we wait for Jesus' return.