Jesus ascended into heaven 40 days after he had been resurrected. During those 40 days he appeared to his followers and continued to teach them. The most detailed account of the ascension is recorded in the Bible's New Testament book of Acts, which was written by Luke:
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 1:1-11, NIV translation.
Read more in the book Jesus the Miracle Worker: The 46 miracles of Jesus explained in chronological order (The Jesus Books) -- now available at amazon.