Ascension Day is possibly the most overlooked church holiday in American Evangelicalism. It is always on a Thursday; it is 40 days after Easter and 10 days before Pentecost, both of which are always on Sundays.
Here's what happened:
When he had led them out to the vicinity of Bethany, he lifted up his hands and blessed them. While he was blessing them, he left them and was taken up into heaven. Then they worshiped him and returned to Jerusalem with great joy. And they stayed continually at the temple, praising God. (Luke 24:50-53)
After his suffering, he showed himself to these men and gave many convincing proofs that he was alive. He appeared to them over a period of forty days and spoke about the kingdom of God. On one occasion, while he was eating with them, he gave them this command: "Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. For John baptized with water, but in a few days you will be baptized with the Holy Spirit."
So when they met together, they asked him, "Lord, are you at this time going to restore the kingdom to Israel?"
He said to them: "It is not for you to know the times or dates the Father has set by his own authority. But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth."
After he said this, he was taken up before their very eyes, and a cloud hid him from their sight.
They were looking intently up into the sky as he was going, when suddenly two men dressed in white stood beside them. "Men of Galilee," they said, "why do you stand here looking into the sky? This same Jesus, who has been taken from you into heaven, will come back in the same way you have seen him go into heaven." (Acts 1:3-11)
Here's why we care:
Q & A 46
Q. What do you mean by saying, "He ascended to heaven"?
A. That Christ, while his disciples watched, was lifted up from the earth to heaven and will be there for our good until he comes again to judge the living and the dead.
Q & A 49
Q. How does Christ's ascension to heaven benefit us?
A. First, he pleads our cause in heaven in the presence of his Father. Second, we have our own flesh in heaven—a guarantee that Christ our head will take us, his members, to himself in heaven. Third, he sends his Spirit to us on earth as a further guarantee. By the Spirit's power we make the goal of our lives, not earthly things, but the things above where Christ is, sitting at God's right hand.
Q & A 50
Q. Why the next words: "and is seated at the right hand of God"?
A. Christ ascended to heaven, there to show that he is head of his church, and that the Father rules all things through him.
Q & A 51
Q. How does this glory of Christ our head benefit us?
A. First, through his Holy Spirit he pours out his gifts from heaven upon us his members. Second, by his power he defends us and keeps us safe from all enemies.