After the Ascension of Christ His disciples stayed in Jerusalem and in one room at the Zion frequently gathered and together they prayed the Lord awaiting impatiently the promised Comforter, The Holy Spirit, who proceeds from the Father. Matthias was voted in to replace Judas among the twelfe.
On the Descent of the Holy Spirit they all were together, when bluster of wind fulfilled the house and tongues of fire appeared and fall upon the apostles. When they came out on the roof of the house, they talked and preached to the gathered people in languages they had not known before, hence all gathered, even the Jews and persons of other faith could understand them. Three thousand people believed on this day the preaching of the apostles and then baptized. This day is considered as the day when the Church of Christ was born.
Even if Pentecost is the privileged feast of the Holy Spirit, the afterfeast and Pentecost Monday stress the importance of the Holy Spirit and its participation in equality as one of the three persons from the Holy Trinity. The importance given to this Monday of the Holy Spirit increased in times when there were wrong ideas being taught about the Holy Spirit and its nature, as if the Holy Spirit was not one with God, not God himself, as he is in the Father and in the Son.
In the New Testament there are many manifestations of the Holy Spirit. During the passage of Pentecost when the apostles took refuge in the Upper Room where they had the Last Supper with Jesus, before his entrance in Jerusalem and death on the cross, they suddenly saw tongues of fire descend upon them. As they left the room they spoke in foreign toungues, considered to be gifts from the Holy Spirit. The Holy Spirit was, and is itself, the promised gift from Jesus, the one that would come as the comforter after his death. After the Resurrection of Jesus, the Orthodox Church’s Pentecost is second in importance alongside eleven other Great Feasts that include the nativity of Jesus, baptism of Jesus, and the ascension and transfiguration.
Pentecost Monday is celebrated religiously in the same way as Pentecost. It is the continuation of the celebration of the feast of Pentecost, and it is the first day of the afterfeast of Pentecost. During the entire week after Pentecost, fasting isn’t permitted.