The Ascension of Christ is described in the Acts of the Apostles (1: 7-11). It took place on the Mount of Olives, 40 days after the Resurrection.
During this period Christ had appeared on a number of occasions to the apostles, and having gathered them together he went with them to the Mount of Olives, where he spoke his final words to them and was then lifted up on a cloud 'which took him out of their sight'.
While they stood amazed, staring up into heaven, they were addressed by two men in white, who told them that Jesus had been taken up into heaven in the same way that they had seen him borne up from earth.
This was followed by the Pentecost.