As has been stated, the feast of Epiphany traditionally (as in from the earliest days of the Church) commemorated three events, all of which relate to the Manifestation of Our Lord to the gentiles, to wit:
the coming of the Magi, the Baptism of Our Lord, and His first miracle at Cana. Many writings of the saints attest to this, and were included in the traditional Divine Office–Sts. Maximus, Jerome, Augustine, and Gregory among them.
A separate liturgical day commemorating the feast of Our Lord’s Baptism did not appear until 1955, when the first in a series of calendar reforms were made in the Church. So the feast is a very modern one. The octave day of the Epiphany had always had the Lord’s Baptism as the gospel of the day, as again, that is part of what Epiphany was about. But in 1955, the octave of Epiphany was suppressed, as were all octaves other then Christmas, Easter, and Pentecost. But, perhaps somewhat ironically, the gospel of the day, and some of the other propers, were preserved, and the octave day, 13 January, was retitled the Commemoration of the Baptism of Our Lord. It remained there until 1970, when the new missal and calendar took effect. That is why you see it in the 1962 missal.
One could make conjecture and say that this was an early example of ecumenical thinking regarding baptism. Up until the post-Vatican II reforms, converts from Protestantism were baptized conditionally upon their reception into the Catholic Church. By the time RCIA was re-institiuted a few years after, the emphasis on a baptism common to all Christians had come in, and there was no more conditional baptism in most receptions. One cannot help but notice the the introduction of the separate baptism feast as a related antecedent.
