The story of the ascension is found in both the Gospel of Luke and the Acts of the Apostles [also written by Luke]. As we know, Jesus appeared to many of his disciples during the 40 days following his resurrection and, on the 40th day, returned with the Apostles to the Mount of Olives where he instructed them to wait in Jerusalem for the coming of the Holy Spirit. In their presence he ascended into the clouds. Two angels then appeared and declared that Jesus would return in glory.
According to Augustine of Hippo, one of the early church fathers, the Feast of Ascension originated with the Apostles and, thus, is one of the oldest feasts practiced by the Church.
The lections may be found here.