The feast of the Ascension is the day commemorating Christ's return to heaven forty days after his Resurrection. It is observed on the Thursday before the Seventh Sunday of Easter, and is a holy day of obligation in the universal Church.

The Ascension is the final event of Jesus' visible earthly ministry. The Feast of the Ascension may be regarded as the day on which Jesus, in his glorified body, took his seat at the right hand of God. This feast may also be interpreted metaphorically as an expression of the enduring relationship between the transcendent Jesus and the community he left behind. It was through the Ascension that Jesus' supremacy over creation was revealed; it was the Ascension, too, which made possible the sending of the Holy Spirit on Pentecost. Just as the Resurrection expressed the reality of Jesus' triumph over death, revealing Jesus as the Risen Christ, so the Ascension assures believers that Jesus is eternally present with God. After the Ascension, Jesus was no longer present among his followers in a physical sense; paradoxically, the ascended Jesus is forever present in the world.

From the Encyclopedia of Catholicism