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 St. Paul’s First Letter to the Corinthians (16:8). It is the 50th day after Easter (if we count both Easter and Pentecost), and it supplants the Jewish feast of Pentecost, which took place 50 days after the Passover and which celebrated the sealing of the Old Covenant on Mount Sinai.
That is why Pentecost is often called “the birthday of the Church.” On this day, with the descent of the Holy Spirit, Christ’s mission is completed, and the New Covenant is inaugurated. It’s interesting to note that St. Peter was already the leader and spokesman for the Apostles on Pentecost Sunday.
In years past, Pentecost was celebrated with greater solemnity than it is today. In fact, the entire period between Easter and Pentecost Sunday was known as Pentecost (and it still is called Pentecost in the Eastern churches, both Catholic and Orthodox). During those 50 days, both fasting and kneeling were strictly forbidden, because this period was supposed to give us a foretaste of the life of Heaven. In more recent times, parishes celebrated the approach of Pentecost with the public recitation of the Novena to the Holy Ghost.