And behold, I am sending the promise of my Father upon you. But stay in the city until you are clothed with power from on high.”

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. And they worshiped him and returned to Jerusalem with great joy, and were continually in the temple blessing God. (ESV)

The Ascension Day is one of the great feasts of the Christian year.  At this time, our Lord's Ascension into Heaven is commemorated.  His earthly ministry -- from His birth, death, burial, and resurrection, are complete and so He is taken into Heaven to sit at the right hand of God the Father.  There He reigns to this very day!