Coming 50 days after Easter and ten days after the Ascension of Our Lord, Pentecost marks the descent of the Holy Spirit on the apostles. For that reason, it is often called the “the birthday of the Church.”
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). 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.
Pentecost 101 from ThoughtCo.
Image: “Pentecost” by Jean II Restout (1732 AD)