The Ascension strikes me as a feast of grief. The apostles stand staring up at where they last saw Jesus because once they move away from where they last saw him, he's really gone. We watch the spot where we last saw a friend. Until the apostles move, there's still a sense of physical connection with Jesus, just as someone might sit beside a dead friend, or remain in church after a funeral, or linger at a graveside. There's a faint, wild hope that he will reappear, say it was a test or that that the Father sends him back because he knows how much they need him. When they move from the spot, the thread is snapped.
The thread does snap, but in a way they least expect. An angel shows up. We think of angels as fantastic, ethereal creatures, but in the Bible, they're as no-nonsense as you get. The angel has no patience with humans associating a physical position with proximity. "Men of Galilee, why are you standing there looking into the sky?" Which is a strange question, seeing as the angel next says that Jesus will return exactly the same way they saw him go. Humanly speaking, if he's coming back that way, it makes sense to stand looking into the sky, what? But the angels know that Jesus just told the apostles that they would be his witnesses starting in Jerusalem. You can't witness in Jerusalem if you're staring up at the sky on Mount Olivet.
So they go back to Jerusalem to pray in the upper room. In the week and so before the Spirit comes on Pentecost (and they didn't know when the Spirit was coming, if they even exactly understood that the Spirit was coming), they do what makes human sense for an organization: they fill empty leadership slots. Out of 120 people (minus twelve for the disciples), they winnowed the field down to two men who fit the criteria of having been with the brethren throughout the whole of Jesus's ministry, Joseph and Matthias. Here the apostles show a first glimmer of wisdom. They don't have the fullness of the Spirit yet. Their prudential judgment has always left something to be desired. So instead of trusting to themselves, they prayed and then cast lots, leaving the choice to the Lord. And then everyone accepts the outcome, with no bickering -- these, the men who jostled for position, who asked Jesus if they could sit at his right hand, who walked right behind Jesus while debating who was the top dog. Now they're ready to receive the Spirit, and they receive the Spirit with such power because their hearts are now ready and open.