The Ascension of Jesus is the penultimate event of the Easter season: In the earlier calendar of the Roman church, Ascension was celebrated 40 days after Easter. Since all faithful Catholics were required to observe the Feast by attending Mass, it has been recently decided to hold the Feast on the Sunday following the Thursday, to relieve Catholics of the burden of finding time on Thursday to attend Mass.

An account of the Ascension can be found in Luke 24:50-51:
Then he led them out as far as Bethany, and lifting up his hands he blessed them. While he blessed them, he parted from them and was carried up into heaven.
Also, in Acts 1:9:
And when he had said this, as they were looking on, he was lifted up, and a cloud took took him out of their sight.
Brief descriptions can also be found in Mark 16:19-20 and John 14:2-6 and in other places.

The purpose of Ascension is given in John 14:2-3:
In my Father's house are many rooms; if it were not so, I would have told you. I am going there to prepare a place for you. And if I go and prepare a place for you, I will come back and take you to be with me that you also may be where I am.
More important to the success of his Church, he explains in John 16:6-8
Because I have said these things, you are filled with grief. But I tell you the truth: It is for your good that I am going away. Unless I go away, the Counselor will not come to you; but if I go, I will send him to you. When he comes, he will convict the world of guilt in regard to sin and righteousness and judgment.
Another name for the Counselor is the Holy Spirit or the Paraclete.