Pentecost, on the 50th day after Easter the Holy Spirit descended on the Apostles, who had gathered together in one place. "And suddenly there came a sound from Heaven as of a rushing mighty wind, and it filled 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 Ghost, and began to speak in other tongues, as the Spirit gave them utterance." (Acts of the Apostles 2:1-13). Through this event the prophecy of a First Testament "And it shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and daughters shall prophesy" (Joel 2:28) is realized.
Some artists depicted Mary, others Peter, at the centre of the Apostles.
See: Duccio di Buoninsegna. Maestà: The Descent of the Holy Spirit.
Anthony van Dyck The Descent of the Holy Spirit (Pentecost).
Giotto. The Pentecost.
El Greco Pentecost.
Anton Raphael Mengs. Pentecost.
Mikhail Vrubel. Pentecost.