The Feast of the Ascension of our Lord God and Savior Jesus Christ is celebrated each year on the fortieth day after the Great and Holy Feast of Easter. Since the date of Easter changes each year, the date of the Feast of the Ascension changes. The Feast is always celebrated on a Thursday.
The Feast itself commemorates when, on the fortieth day after His Resurrection, Jesus led His disciples to the Mount of Olives, and after blessing them and asking them to wait for the fulfillment of the promise of the Holy Spirit, He ascended into heaven.
The story of the Ascension of our Lord is found in the book of the Acts of the Apostles 1:3-11. It is also mentioned in the Gospels of Mark (16:19) and Luke (24:50-53). The moment of the Ascension is told in one sentence: "He was lifted up before their eyes in a cloud which took Him from their sight" (Acts 1:9).
Christ made His last appearance on earth, forty days after His Resurrection from the dead. The Acts of the Apostles states that the disciples were in Jerusalem. Jesus appeared before them and commanded them not to depart from Jerusalem, but to wait for the "Promise of the Father". He stated, "You shall be baptized with the Holy Spirit not many days from now" (Acts 1:5).
After Jesus gave these instructions, He led the disciples to the Mount of Olives. Here, He commissioned them to be His witnesses "in Jerusalem, and in all Judea and Samaria, and to the end of the earth" (Acts 1:8). It is also at this time that the disciples were directed by Christ to "go and make disciples of all nations, baptizing them in the name of the Father and the Son and the Holy Spirit" (Matthew 28:19). Jesus also told them that He would be with them always, "even to the end of the world" (Matthew 28:20).
As the disciples watched, Jesus lifted up His hands, blessed them, and then was taken up out of their sight (Luke 24:51; Acts 1:9). Two angels appeared to them and asked them why they were gazing into heaven. Then one of the angels said, "This same Jesus, which is taken up from you into heaven, shall so come in like manner as you have seen Him going into heaven" (Acts 1:11).
The Church’s celebration of the ascension, as all such festal celebrations, is not merely the remembrance of an event in Christ’s life. Indeed, the ascension itself is not to be understood as though it were simply the supernatural event of a man floating up and away into the skies. The holy scripture stresses Christ’s physical departure and His glorification with God the Father, together with the great joy which His disciples had as they received the promise of the Holy Spirit Who was to come to assure the Lord’s presence with them, enabling them to be His witnesses to the ends of earth (Lk 24.48–53; Acts 1.8–11; Mt 28.20; Mk 16.16–14).
In the Church the believers in Christ celebrate these very same realities with the conviction that it is for them and for all men that Christ’s departure from this world has taken place. The Lord leaves in order to be glorified with God the Father and to glorify us with himself. He goes in order to “prepare a place” for and to take us also into the blessedness of God’s presence. He goes to open the way for all flesh into the “heavenly sanctuary . . . the Holy Place not made by hands” (see Hebrews 8–10). He goes in order send the Holy Spirit, Who proceeds from the Father to bear witness to Him and His gospel in the world, making Him powerfully present in the lives of disciples.
The hymns of the feast of the Ascension, written by St. Nerses of Lambron (1153 - 1198) sing of all of these things.
“Our Lord ascended into heaven, telling the disciples, Stay in the city of Jerusalem until you shall be arrayed in might from on high. The apostles agreeing in thought, religion and speech stayed in the holy Upper Room and awaited the coming of the Holy Spirit.” (Orhnutyoon)
Adapted from different sources