The Baptistm of the Christ, is a feast day celebrating the baptism of Jesus Christ in the Jordan River by Saint John the Baptist. In the Roman rite, the feast was originally celebrated together with the coming of the Magi and the wedding at Cana. As time progressed, the feast became overshadowed by the other elements of the Epiphany and so Pope Pius XII instituted a separate commemoration specifically for the Baptism. When the feast was instituted under the demonication "Commemoration of the Baptism of our Lord", it was listed for January 13th as a major double. Five years later, Pope John XXIII kept the date but moved the Commemoration to the rank of secon-class feast.
Pope Paul VI then set the date for the feast as the first Sunday after January 6, unless the the Epiphany is celebrated on January 7 or 8, in which case it would be the following Monday. Saint Pope John Paul II began a tradition of the Pope baptizing babies in the Sistine Chapel on the feast day as well. The Baptism of the Lord is recorded in the Gospels of Mark, Matthew and Luke:
9 At that time Jesus came from Nazareth in Galilee and was baptized by John in the Jordan. 10 Just as Jesus was coming up out of the water, he saw heaven being torn open and the Spirit descending on him like a dove. 11 And a voice came from heaven: “You are my Son, whom I love; with you I am well pleased.”
13 Then Jesus came from Galilee to the Jordan to be baptized by John. 14 But John tried to deter him, saying, “I need to be baptized by you, and do you come to me?” 15 Jesus replied, “Let it be so now; it is proper for us to do this to fulfill all righteousness.” Then John consented. 16 As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him. 17 And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”
21 When all the people were being baptized, Jesus was baptized too. And as he was praying, heaven was opened 22 and the Holy Spirit descended on him in bodily form like a dove. And a voice came from heaven: “You are my Son, whom I love; with you I am well pleased.”