The Ascension of Our Lord and Saviour Jesus Christ is one of the Great Feasts of the Orthodox Church, celebrated forty days after Pascha (and thus always falling on a Thursday).
The first account of the Ascension found in the Bible is in the Gospel of Mark (16:14-19). The description is brief. Jesus and the remaining eleven disciples are seated at a table, presumably in a room in or near Jerusalem. Jesus commands his followers to spread the Gospel, and that those who believe will be known by their invulnerability to poison, ability to heal the sick, and the like. After delivering these final words, Jesus is received into heaven to sit at the right hand of God. No description of the Ascension itself is given; Mark simply states that it happened.
The Gospel of Luke is even more brief in its description (24:50-51). Jesus led the eleven to Bethany, not far from Jerusalem. While in the act of blessing them, Jesus was carried up to heaven.
The third, and most celebrated account of the Ascension is in the Acts of the Apostles (1:9-12). For forty days after the Resurrection, Jesus continued to preach the Gospel. Jesus and the eleven were gathered near Mt. Olivet (or the Mount of Olives), to the northeast of Bethany. Jesus tells his disciples that they will receive the power of the Holy Spirit and that they will spread his message the world over. Jesus is taken up and received by a cloud. Some traditions say that he was taken up in a fiery chariot, much like the Prophet Elijah. Two men clothed in white appear and tell the disciples that Jesus will return in the same manner as he was taken. They say: "Ye men of Galilee, why stand ye gazing up into Heaven? This same Jesus, Who is taken up from you into Heaven, shall so come in like manner as ye have seen Him go into Heaven" (Acts 1:11). Afterwards, the disciples return to Jerusalem rejoicing, remaining continually in the Temple.
The Gospel of Matthew ends at a mountain in Galilee, with Jesus commanding the disciples to spread the Gospel. No mention of the Ascension is made.
The Ascension of Christ shows the last stage in God's plan for mankind: total union with Himself upon one's departure from the world. In the Ascension resides the meaning and the fullness of Christ's Resurrection....and with Christ, man's nature ascends also.
O Christ God, Thou hast ascended in Glory,
Granting joy to Thy disciples by the promise of the Holy Spirit.
Through the blessing they were assured
That Thou art the Son of God,
The Redeemer of the world!
Fifty days after the Resurrection, on the excising Jewish feast of Pentecost, while the disciples and many other followers of Jesus Christ were gathered together to pray, the Holy Spirit descended upon them in the form of "cloven tongues of fire," with the sound of a mighty rushing wind, and they began to speak in languages that they did not know. There were many visitors from the Jewish Diaspora to Jerusalem at that time for the Jewish observance of the feast, and they were astonished to hear these untaught fishermen speaking praises to God in their alien tongues. The number fifty, as in the fiftieth day after Pascha, stands for eternal and heavenly fulfilment, seven times seven, plus one.
The Orthodox Church sees Pentecost as the final fulfilment of the mission of Jesus Christ and the first beginning of the messianic age of the Kingdom of God, mystically present in his Church. It is traditionally called the beginning of the One Holy Catholic and Apostolic Church.
Besides celebrating the coming of the Holy Spirit, the feast also celebrates the full revelation of the divine Trinity, Father, Son, and Holy Spirit. Hymns of the Church, celebrate the sign of the final act of God's self-disclosure to the world of His creation.
To Orthodox Christians, the feast of Pentecost is not just a celebration of an event in history. It is also a celebration their membership in the Church. They have lived Pentecost and received "the seal of the gift of the Holy Spirit" in the sacrament of chrismation.
Blessed art Thou O Christ Our God.
Thou hast revealed the fishermen as most wise,
By sending down upon them the Holy Spirit.
Through them Thou didst draw the world into Thy net.
O Lover of Man, Glory to Thee!