The first Pentecost feast took place 50 days after Passover. In the Old Testament, Pentecost celebrated God’s gift of the 10 commandments to Moses. In the New Testament, the Pentecostal feast took on new meaning as the coming of the Holy Spirit to Jesus' disciples, as he had foretold them. This was the final fulfillment of Jesus' mission and the beginning of the disciples preaching about Jesus as the risen Christ, also known as the birthday of the Church.
The word "Pentecost" comes from the Greek word "Pentekostos," meaning "fifty." The Orthodox tradition also refers to Pentecost Sunday as Trinity Day, because the coming of the Holy Spirit was the fulfillment of the divine Trinity revelation: Father, Son and Holy Spirit. Acts 2 in the New Testament depicts the coming of the Holy Spirit to the disciples as a sound from heaven like a mighty rushing wind and tongues of fire resting upon each one of them.
Pentecost marked the "birthday" of Christianity, because when Peter preached after the coming of the Holy Spirit, 3,000 new believers were baptized.
Some scholars believe that Luke's story in Acts 2 was referring to an experience of speaking in tongues, or praise in an unknown language. Others state that the listeners understood the disciples' message.
Modern day Pentecostals get their name because of their emphasis on the gifts of the Spirit and the experience of a direct personal experience of God like the first disciples had, often manifested as speaking in tongues.