Pentecost

Pentecost (literally means "fifty") is the Greek name for the Jewish Festival of Weeks. Christians started using this term from the Greek speaking Jews who used the phrase pentekoste hemera, meaning "fiftieth day" as the Festival of Weeks was to start fifty days after the end of Passover. Originally this time was a harvest festival, but in time the Jews used the day to also commemorate the giving of the Law on Mt. Sinai.

Old Testament
See Festival of Weeks

New Testament
After the ascension of Jesus, His followers were gathered in a house in Jerusalem on the day of Pentecost when suddenly a strong wind rushed through the dwelling. Tongues of fire appeared on each of them and they were filled with the Holy Spirit causing them to speak in various languages. Because of Pentecost, there were a great multitude of Jews from every nation in the city. When they heard the commotion they gathered together around the house and heard their own languages being spoken and were both amazed and perplexed. Some even accused the followers of Jesus of being drunk.

It was at this time that Peter, stood with the other disciples, and gave the first Christian sermon on record to those who had gathered. Peter based his sermon on the writings of the prophet Joel who promised that the Spirit will be poured out on all flesh. This would be a sign of the coming of the "day of the Lord." Peter explained to the crowd that the Spirit has now been poured out in fullfillment of God's promise. Upon hearing this the multitude asked Peter what they should do. He urged them to repent and be baptized in the name of Jesus. Luke reports that about 3,000 people were added to the church that day.