Ascension Day celebrates Jesus’s ascension to heaven after he was resurrected on Easter Day.
A quotation from Mark 16:9-20 tells the story.
He appeared first to Mary of Magdala. She went and carried the news to his mourning and sorrowful followers, but when she told them that he was alive they did not believe her.
Later he appeared to two of the disciples as they were walking into the countryside. They also went and took the news to the others, but again they did not believe that the Lord was alive.
Then, when the eleven disciples were at the table. He appeared to them and reproached them because they had not believed those who had seen him after he was raised from the dead.
Then he said to them: ‘Go forth to every part of the world, and proclaim the good news to the whole creation. Those who believe it and receive baptism will find salvation; those who do not believe will be condemned, Faith will bring with it these miracles: believers will cast out devils in my name and speak in strange tongues; if they handle snakes or drink any deadly poison, they will come to no harm; and the sick on whom they lay their hands will recover.’
So after talking with them the Lord Jesus was taken up into heaven, and he took his seat at the right hand of God.
Jesus’s prophecy in this passage is believed to foreshadow the later events of Pentecost.