The feast of the Holy Trinity is called Pentecost because the descent of the Holy Spirit upon the Apostles was accomplished on the fiftieth day after the Resurrection of Christ. The feast of Christian Pentecost includes a double celebration: - and to the glory of the Most Holy Trinity, and to the glory of the Most Holy Spirit, apparently descended upon the Apostles and sealed the new eternal covenant of God with men.
The first day of Pentecost, ie. Sunday, the Church dedicates mainly to the glory of the Most Holy Trinity; and this day in the people is called the Trinity Day, and the second, ie, Monday - for the glory of the Holy Spirit, which is why it is called the Spirit Day.
Celebration of the Holy Spirit, the Church begins, as usual, evening worship in Trinity day. On this service, with kneeling, three touching prayers of Basil the Great are read, in which we confess our sins to the Heavenly Father and, for the sake of the great sacrifice of His Son, we ask for pardons; we also ask the Lord Jesus Christ to grant us the Divine Spirit, to enlighten and affirm our souls, and, finally, we pray for the deceased fathers and our brothers, may the Lord bless them in a place of light, evil, and peace.
In the Feast of Pentecost, it is customary to decorate the temple and its dwellings with tree branches and flowers, and stand in the temple with flowers in hand. The decoration of churches and dwellings on this day with greens and flowers is, first, the confession of the life-giving force of the Life-giving Spirit; and secondly, by the proper dedication to Him of the first spring.
After the ascension of Jesus Christ, the tenth day came: it was the fiftieth day after the Resurrection of Christ. The Jews then had a great feast of Pentecost in memory of the Sinai legislation. All the apostles, together with the Mother of God and with the other disciples of Christ and the other believers, were unanimously in the same room in Jerusalem. It was the third hour of the day, according to the Jewish account of the clock, that is, according to ours - the ninth hour of the morning. Suddenly there was a noise from the sky, as if from a rushing strong wind, and filled the whole house where the disciples of Christ were. And tongues of fire appeared and rested (stopped) one on each of them. All were filled with the Holy Spirit and began to praise God in different languages, which they did not know before. Thus the Holy Spirit, according to the promise of the Savior, descended into the apostles, in the form of fiery tongues, as a sign that He gave the apostles the ability and power to preach the teaching of Christ to all nations; descended in the form of fire as a sign that he has the power to singe out sins and purify, sanctify and warm souls.
On the occasion of the feast of Pentecost, in Jerusalem at this time there were many Jews who came from different countries. Hearing the noise, a huge crowd of people gathered near the house where the disciples of Christ were. Everyone in the people was amazed and asked each other: "Are not they all Galileans? How do we hear each of our own languages in which they were born?" How can they speak our languages about the great works of God? " And in bewilderment they said: "they got drunk sweet wine".
Then the apostle Peter, having risen together with the other eleven apostles, said that they were not drunk, but that the Holy Spirit descended upon them, as was prophesied by the prophet Joel, and that Jesus Christ, whom the Jews crucified, rose from the dead, ascended to heaven and poured out on them the Holy Spirit. Concluding the sermon on Jesus Christ, the apostle Peter said: "Now, firmly know, all the people of Israel, that God sent this Jesus, the Savior and Christ, whom you crucified."
The preaching of Peter so affected the listeners that very many believed in Jesus Christ. They began to ask Peter and the other apostles: "What shall we do, men and brethren?"
Peter answered them: "Repent and be baptized in the name of Jesus Christ for the remission of sins, then you will also receive the gift of the Holy Spirit."
Those who believed in Christ willingly accepted baptism, there were about three thousand such people on this day. Thus the kingdom of God began to be established on earth, that is, the holy Church of Christ.
Since the day of the descent of the Holy Spirit, Christian faith has spread rapidly, with the help of God; the number of believers in the Lord Jesus Christ grew from day to day. Being taught by the Holy Spirit, the apostles boldly preached to all about Jesus Christ, the Son of God, His sufferings for us and the resurrection from the dead. The Lord helped them with the great many miracles that were performed through the apostles in the name of the Lord Jesus Christ. Initially, the apostles preached to the Jews, and then dispersed to different countries for preaching to all nations. To perform the sacraments and preach the Christian teaching, the apostles delivered through the ordination of bishops, presbyters (priests or priests) and deacons.
The grace of the Holy Spirit, which was explicitly taught to the apostles in the form of fiery tongues, is now presented in our Holy Orthodox Church in invisible - in its holy ordinances, through successors-apostles - pastors of the Church - bishops and priests.
NOTE: See Acts of St. Apostles, ch. 2, ch. 14, 23.