Peter and the other apostles replied:
"We must obey God rather than men!
The God of our fathers raised Jesus from the dead—
whom you had killed by hanging him on a tree.
God exalted him to his own right hand
as Prince and Savior
that he might give repentance
and forgiveness of sins to Israel.
We are witnesses of these things,
and so is the Holy Spirit,
whom God has given to those who obey him."
John Jansen has a nice article on the theology of the Ascension that's well worth reading. At Intellectuelle there is a fragment from a sermon by Augustine on the subject. Magic Statistics has the collect for today from the 1662 Book of Common Prayer.
The heart of the feast of the Ascension is mediation; it is about Christ's role as mediator between God and man, and about His intercession in glory for us. Therefore there is no better way to celebrate the day than conciliation and reconciliation with our fellow human beings and intercession before God for them.