Ascension Day commemorates the ascension of Jesus into heaven following His crucifixion and resurrection. During the forty day period after His resurrection, He preached and intermingled with his apostles and disciples.
Luke 24:49-53
Jesus said to his disciples, "See, I am sending upon you what my Father promised; so stay here in the city until you have been 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 was blessing them, he withdrew from them and was carried up into heaven. And they worshiped him, and returned to Jerusalem with great joy; and they were continually in the temple blessing God.
Mark 16:9-15,19-20
Now after Jesus rose early on the first day of the week, he appeared first to Mary Magdalene, from whom he had cast out seven demons. She went out and told those who had been with him, while they were mourning and weeping. But when they heard that he was alive and had been seen by her, they would not believe it.
After this he appeared in another form to two of them, as they were walking into the country. And they went back and told the rest, but they did not believe them. Later he appeared to the eleven themselves as they were sitting at the table; and he upbraided them for their lack of faith and stubbornness, because they had not believed those who saw him after he had risen. And he said to them, "Go into all the world and proclaim the good news to the whole creation.
So then the Lord Jesus, after he had spoken to them, was taken up into heaven and sat down at the right hand of God. And they went out and proclaimed the good news everywhere, while the Lord worked with them and confirmed the message by the signs that accompanied it.