One of the main Christian holidays - Holy Trinity Day - is celebrated on the 50th day after Easter. In Orthodoxy, it is one of the twelve feasts and glorifies the Holy Trinity. And it is called Pentecost because the descent of the Holy Spirit on the Apostles took place on the fiftieth day after the Resurrection of Christ.
After the ascension of Jesus Christ, the tenth day came: it was the fiftieth day after the Resurrection of Christ. The Jews had a great feast of Pentecost to commemorate the laws of Sinai. All the apostles, together with the Mother of God and with other disciples of Christ and other believers, were unanimously in the same upper room in Jerusalem.
“It was the third hour of the day, according to the Jewish account of hours, that is, in our opinion, the ninth hour of the morning. Suddenly there was a noise from heaven, as if from a rushing strong wind, and filled the whole house where the disciples of Christ were. And fiery tongues appeared and rested (stopped) one on each of them. Everyone was filled with the Holy Spirit and began to praise God in different languages, which they did not know before.