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.
Acts 2:1-4 (New International Version)
Before he ascended into heaven, Christ told his followers to stay in Jerusalem until they were "clothed with power from on high." [Luke 24:49]
Pentecost was the Greek name for the Feast of Weeks, celebrated by the Hebrews in recognition of the law given on Mt. Sinai. In Judaism, the feast is still celebrated as Shavuot.
But in Christian tradition, and the Christian liturgical year, Pentecost Sunday became the day to recognize the coming of the Holy Spirit upon the Apostles and followers of Jesus. It is traditionally celebrated seven weeks (50 days) after Easter Sunday. It follows ten days after the Ascension.
Dr. Stephen Seamans, professor of Christian Doctrine at Asbury Theological Seminary in Kentucky, recommends Five Books on the Holy Spirit.
According to legend, King Arthur always gathered all his knights at the round table for a feast and a quest on Pentecost:
So ever the king had a custom that at the feast of Pentecost in especial, afore other feasts in the year, he would not go that day to meat until he had heard or seen of a great marvel. [Le Morte d'Arthur, Thomas Malory. Book 7, chapter 1]
Pentecost is sometimes known as "the birthday of the Church."