This Sunday is the Day of Pentecost, the 50th day after Easter, the celebration of the coming of the Holy Spirit, the "birthday" of the Christian Church.
"Pentecost" comes from the Greek word pentekostos, meaning fiftieth, from which one of the most important feasts in the Jewish calendar derives its name. Fifty days after Passover, the Jews celebrated the "Feast of Harvest" or "Feast of Weeks".
This became one of the great pilgrimage feasts of Judaism, as scattered Jews returned to Jerusalem for worship.
Since the second century, Christians have celebrated the coming of the Holy Spirit upon the disciples fifty days after the death and resurrection of Jesus, on the feast of Pentecost, as the birthday of the church. After Christmas and Easter, Pentecost marks the most important celebration on the Christian calendar. Happy Birthday, Church!