The word “Pentecost” has its origins in Greek and it means “fiftieth.” Fifty days after Easter Sunday, we commemorate the descent of the Holy Spirit upon the early disciples.
However, Pentecost was a Jewish feast before it was a Christian feast. On the fiftieth day after Passover, the Jews celebrated their covenant relationship with God by recalling Moses receiving the law on Mount Sinai. Just as the Gospel accounts reinterpret the significance of the meal celebrated on Passover by telling the story of Jesus instituting the Eucharist, so the Acts of the Apostles reinterprets the significance of the feast of Pentecost, making it a celebration of the gift of the Spirit to the Church.
Because the arrival of the Holy Spirit enabled the apostles to launch their earthly ministry to make disciples of all nations, Pentecost is also celebrated as the birth of the Church. From that point on, the apostles carried the message of Christ to the whole world.