1. The Resurrection/Ascension - At the moment Jesus was raised from the dead he went, not back to earth, but to the Father. He ascended to the father, and it is from there that he appeared to the disciples. (Just as his Resurrection was not witnessed by anyone, neither was this Ascension.)
2. The End of Special Appearances - Jesus, after his Resurrection/Ascension, was no longer limited by time and space. He was and is present to his followers even more closely than before. in the days after his Resurrection/Ascension, Jesus manifested himself to the disciples at various time and places in an extraordinary, visible way.
At some point, these special appearances came to an end. Luke dramatizes the end of these extraordinary visible appearances by describing Christ visibly ascending to heaven.
It is Ascension in this sense that is celebrated in some dioceses today.
- Little White Book, Diocese of Saginaw