After he rose from the dead, Jesus appeared to his disciples for 40 days. At the end of this time he told his followers that it was necessary for him to return to the Father. Jesus promised he would not leave them alone and would send the Advocate, the Holy Spirit, to be with them. He gave them a commission which is found at the end of St. Matthew’s Gospel,
“Go therefore and make disciples of all nations, baptizing them in the name of the Father, and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold I am with you always until the end of the age.” Matthew 28: 19-20
Ascension Thursday is celebrated this year on Thursday, May 5.