The Feast of Holy Pentecost is celebrated each year on the fiftieth day after the Great and Holy Feast of Pascha (Easter) and ten days after the Feast of the Ascension of Christ. The Feast is always celebrated on a Sunday.
The Feast commemorates the descent of the Holy Spirit upon the Apostles on the day of Pentecost, a feast of the Jewish tradition. It also celebrates the establishment of the Church through the preaching of the Apostles and the baptism of the thousands who on that day believed in the Gospel message of salvation through Jesus Christ. The Feast is also seen as the culmination of the revelation of the Holy Trinity.
The story of Pentecost is found in the book of The Acts of the Apostles. In Chapter two we are told that the Apostles of our Lord were gathered together in one place. Suddenly, a sound came from heaven like a rushing wind, filling the entire house where they were sitting. Then, tongues of fire appeared, and one sat upon each one of Apostles. They were all filled with the Holy Spirit and began to speak in other languages as directed by the Spirit (Acts 2:1-4).
This miraculous event occurred on the Jewish Feast of Pentecost, celebrated by the Jews on the fiftieth day after the Passover as the culmination of the Feast of Weeks (Exodus 34:22; Deuteronomy 16:10). The Feast of Weeks began on the third day after the Passover with the presentation of the first harvest sheaves to God, and it concluded on Pentecost with the offering of two loaves of unleavened bread, representing the first products of the harvest (Leviticus 23:17-20; Deuteronomy 16:9-10).
Since the Jewish Feast of Pentecost was a great pilgrimage feast, many people from throughout the Roman Empire were gathered in Jerusalem on this day. When the people in Jerusalem heard the sound, they came together and heard their own languages being spoken by the Apostles (Acts 2:5-6). The people were amazed, knowing that some of those speaking were Galileans, and not men who would normally speak many different languages. They wondered what this meant, and some even thought the Apostles were drunk (Acts 2:7-13).
Peter, hearing these remarks, stood up and addressed the crowd. He preached to the people regarding the Old Testament prophecies about the coming of the Holy Spirit. He spoke about Jesus Christ and His death and glorious Resurrection. Great conviction fell upon the people, and they asked the Apostles, “What shall we do?” Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit” (Acts 2:38-39).
The Bible records that on that day about three thousand were baptized. Following, the book of Acts states that the newly baptized continued daily to hear the teaching of the Apostles, as the early Christians met together for fellowship, the breaking of bread, and for prayer. Many wonderful signs and miracles were done through the Apostles, and the Lord added to the Church daily those who were being saved (Acts 2:42-47).
~ Taken from Great Lent, Holy Week & Pascha, Greek Orthodox Archdiocese of America, http://lent.goarch.org/sunday_of_pentecost/learn/.