Acts of the Apostles describes the following scene:
And when the days of the Pentecost were accomplished, they were all together in one place: And suddenly there came a sound from heaven, as of a mighty wind coming, and it filled the whole house where they were sitting. And there appeared to them parted tongues as it were of fire, and it sat upon every one of them: And they were all filled with the Holy Ghost, and they began to speak with divers tongues, according as the Holy Ghost gave them to speak (2:1-4)
Pentecost means 50 days in Greek; it signifies the time between the Resurrection and what came to be the feast of Pentecost, the birthday of the Church. At this moment the Holy Spirit, represented iconographically as a dove, descended upon the disciples and tongues of fire appeared over the apostles' heads. Acts lists Mary as one in the upper room, and many illuminations give her the seat of prominence. In some she even teaches or reads from a book.