The Solemnity of the Ascension of the Lord is Celebrated the Thursday of the sixth week of Easter.
What is the Ascension?
The Ascension refers to the day celebrated 40 days after Easter when Catholics celebrate the fact that Jesus returns to God after being with his disciples after the resurrection. It is a central element of our tradition and is included in the creed recited at Mass each Sunday. It is also a Holy Day of Obligation and Catholics are required to go to mass on that day.
The scriptural basis for this comes from Acts 1:9-11.
And after He had said these things, He was lifted up while they were looking on, and a cloud received Him out of their sight.
And as they were gazing intently into the sky while He was going, behold, two men in white clothing stood beside them. They also said, “Men of Galilee, why do you stand looking into the sky? This Jesus, who has been taken up from you into heaven, will come in just the same way as you have watched Him go into heaven.”
Source: bustedhalo.com