NOTES for Act 1:1-11
How different the behavior of Christ's disciples is before His death and after it! When Jesus told them about His coming death and that He was going to the Father, they were grieved (see John 14:28). Now, witnessing His Ascension, they rejoice instead, even though the Teacher is visibly leaving them: "And while He blessed them, He began to depart from them and be carried up into heaven. They worshiped Him and returned to Jerusalem with great joy." Even though Pentecost had not yet come, the apostles had already lived through an event that turned their whole life upside down. The death and Resurrection of their Teacher changed the heart of each one of them.
In addition, as we read in the Acts of the Apostles, something else happened during the Ascension (see Acts 1:10-11). Two men in white garments appeared to the apostles and announced the Second Coming of the Lord. How amazing that, without waiting, as it were, for the descent of the Spirit who "will teach you all things and remind you of everything" (John 14:26), the Lord Himself after the Resurrection appeared to the disciples "for forty days" and spoke about the Kingdom of God, and now these two men remind them of Jesus' return. Maybe this is the most important thing in the feast of the Ascension: that we are witnesses to the promise of His Second Coming, rejoicing that He goes to the Father.