Pentecost Sunday is one of the most ancient feasts of the Church, celebrated early enough to be mentioned in the Acts of the Apostles (20:16) and Saint Paul's First Letter to the Corinthians (16:8). Pentecost is celebrated on the 50th day after Easter (if we count both Easter Sunday and Pentecost Sunday), and it supplants the Jewish feast of Pentecost, which took place 50 days after the Passover and celebrated the sealing of the Old Covenant on Mount Sinai.
The History of Pentecost Sunday
The Acts of the Apostles recounts the story of the original Pentecost Sunday (Acts 2). Jews "from every nation under heaven" (Acts 2:5) were gathered in Jerusalem to celebrate the Jewish feast of Pentecost. On that Sunday, ten days after the Ascension of Our Lord, the Apostles and the Blessed Virgin Mary were gathered in the Upper Room, where they had seen Christ after His Resurrection:
And suddenly there came from the sky a noise like a strong driving wind, and it filled the entire house in which they were. Then there appeared to them tongues as of fire, which parted and came to rest on each one of them. And they were all filled with the holy Spirit and began to speak in different tongues, as the Spirit enabled them to proclaim. [Acts 2:2-4]
Christ had promised His Apostles that He would send His Holy Spirit, and, on Pentecost, they were granted the gifts of the Holy Spirit. The Apostles began to preach the Gospel in all of the languages that the Jews who were gathered there spoke, and about 3,000 people were converted and baptized that day.
That is why Pentecost is often called "the birthday of the Church." On Pentecost Sunday, with the descent of the Holy Spirit, Christ's mission is completed, and the New Covenant is inaugurated. It's interesting to note that Saint Peter, the first pope, was already the leader and spokesman for the Apostles on Pentecost Sunday (see Acts 2:14ff).
by Scott P. Richert