The tenth day after the Ascension of Jesus Christ was the fiftieth day after the Resurrection of Christ. It was the Jews’ great feast of Pentecost, which commemorated the giving of the Law on Mt. Sinai.
All the apostles, the Mother of God, the other disciples of Christ, and other of the faithful were all together in one room in Jerusalem. It was the third hour of the day by the Hebrew reckoning of hours, according to our system — nine o’clock in the morning. Suddenly a sound came from Heaven like the rush of a mighty wind, and it filled all the house where they were sitting. There descended on them tongues that looked like fire, which rested on each one of them. They were all filled with the Holy Spirit and began to speak languages previously unknown to them. Thus the Holy Spirit, according to the promise of the Saviour, descended on the apostles in the form of tongues of fire as a sign that He gave the apostles the ability and zeal to preach the teachings of Christ to all peoples. He descended in the form of fire as a sign of the power to cleanse sins and to sanctify and warm souls.
On the occasion of the feast of Pentecost, there were in Jerusalem many Jews who had come from various nations. Hearing the noise, a great multitude of people came together around the house where the disciples of Christ were. They were all bewildered and asked each other, "Are not all these who are speaking Galileans? How is that each of us hears his own native language? How are they able to tell in our languages the mighty works of God?" In disbelief they said, "They are filled with new wine."
Then, the Apostle Peter standing with the eleven said that they were not drunk, but the Holy Spirit had descended upon them, and that had been prophesied by the Prophet Joel, and that Jesus Christ, Whom the Jews had crucified, had risen from the dead, ascended into Heaven, and poured out on them the Holy Spirit. Finishing this sermon about Jesus Christ, the Apostle Peter said, "Let all the house of Israel therefore know assuredly that God has made Him both Lord and Christ, this Jesus, Whom you crucified."
The sermon of Peter moved those who heard it so much that many more started to believe in Jesus Christ. They asked Peter and the rest of the apostles, "Brethren, what shall we do?"
Peter said to them, "Repent and be baptized, every one of you in the name of Jesus Christ for the forgiveness of your sins; then, you shall also receive the gift of the Holy Spirit."
So those who believed in Christ readily accepted baptism, and there were added by that day about three thousand souls. Thus, the building of the Kingdom of God on earth, the holy Church of Christ, began.
From the day of the descent of the Holy Spirit the Christian faith quickly began to spread with the help of God, and the number of believers in the Lord Jesus Christ multiplied. Instructed by the Holy Spirit, the apostles preached boldly to all about Jesus Christ, the Son of God, about His suffering for us and resurrection from the dead. The Lord helped them with many great miracles which were performed by the apostles in the name of the Lord Jesus Christ. At first, the apostles preached to the Jews and then dispersed to various countries to preach to all people. To perform the sacraments and to preach Christianity, the apostles established bishops, presbyters, and deacons by the laying of hands on them.
This grace of the Holy Spirit, which was clearly conferred on the apostles in the form of tongues of fire, is now conferred in our Holy Orthodox Church invisibly in its sacraments, through the successors to the apostles, the pastors of the Church, its bishops and priests.
Note: See the Acts of the Apostles 2:14-23.
The Descent of the Holy Spirit on the Apostles is celebrated by the Holy Orthodox Church as one of the greatest feast-days on the fiftieth day after Pascha because the Holy Spirit descended on the Apostles on the fiftieth day after the resurrection of Jesus Christ. It is therefore called Pentecost or Holy Trinity Day; for from this day, the action of the Holy Trinity was revealed to the world, and people learned to venerate and glorify the three Persons of the one God: the Father, and Son, and Holy Spirit.
On this feast-day it is customary to decorate the church and its hall with greenery and to hold flowers during the Liturgy expressing thus our joy and thanksgiving to God for His Holy and Life-giving Spirit, which renews people and gives them birth through baptism into a new life. The flowers and greenery signify life.
The day following Holy Trinity Day is called Spirit Day and is devoted to glorification of the Holy Spirit.
Troparion of Pentecost.
Blessed art Thou, Christ our God, Who hast shown forth the fishermen as supremely wise, by sending down upon them the Holy Spirit, and through them, didst draw the world into Thy net. O Lover of mankind, glory be to Thee.
Taken from The Law of God By Archpriest Seraphim Slobodskoy