Pentecost is the 50th day after Easter (hence the name, pentecost means the 'the fiftieth day') and celebrates the descent of the Holy spirit upon the Apostles of Christ, Mary and others. The story is told in The book of Acts:
"And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Spirit, and began to speak with other languages, as the Spirit gave them utterance. And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven. Now when this was noised abroad, the multitude came together, and were confounded, because that every man heard them speak in his own language." (Acts 2:1–6)
The Holy spirit (usually shown as a dove is art) is the power of God in action, his active force, his energy. Here on this engraving the Holy Spirit can be seen at the top, the descending white dove. The small flames on the heads of the gathered group below is the received blessing of the Holy Spirit. Between the group and the Holy spirit are representations of Science, Theology and Philology.