To clarify the matter, the best method is to go to the source of the feast, which is the historical baptism of Jesus marking the onset of his ministry. Scripture scholars are in general agreement that the baptism of Jesus is one of the events about which we have the most historical certainty. Using what is called the "law of embarrassment," scholars generally place a high premium on those events in Jesus’ life that might be misconstrued as embarrassing; on the ground that no Christian writer, particularly the evangelists, would have invented something that casts the Savior in a diminutive light. The idea of Jesus submitting himself to the baptism of John was at the very least a great puzzlement to the early Church. (St. Matthew, in his gospel, goes so far as to smooth things over by inventing a private conversation between Jesus and John; however St. Matthew's account is written considerably later than the original, that of St. Mark.)
To establish the meaning of the Baptism of Jesus more distinctly, I referr to one of the United States' greatest Catholic scholars in our generation, Father John Meier. Father Meier devotes considerable scholarship to the relationship of Jesus and John the Baptist. He concludes that there are three things we can safely assume about this complicated relationship. The first is that Jesus did spend some time as a disciple of John, and that the Baptist's teaching had a significant effect upon Jesus. The second is that Jesus did in fact undergo a baptism at the feet of John, for the same purpose that all of John's other disciples and followers did: a washing away of a past life and a change in direction in anticipation of a final day of judgment. And finally, at least at this stage of his life, Jesus did believe in a dramatic intervention by God in the future in which the world would be reordered and the glory of God established in a glorious but unspecified way.
It is also important to note the time of Jesus’ baptism: the beginning of his public ministry where he would go forth working signs and wonders as indications that the Kingdom of God was approaching. This is the reason that the Church closes the Christmas Season with the observance of Jesus’ baptism. The feast marks the end of the formative years of Jesus and the beginning of his active ministry.
Clearly this is a very rich feast. It may be disconcerting to some who maintain that Jesus had instant full cognizance of his human and divine natures from the first moment of awareness. The idea that his full self understanding of his mission developed over time, in the same fashion as all other human beings may come as a surprise, although in his gospel St. Luke's does tell us early that the young Jesus grew in age, wisdom, and grace.
In my own life as a catechist I have teased a number of classes by asking if Jesus in fact needed to be baptized. The hesitation and struggle of students to raise their hands to an either yes or no commitment is a bit amusing to watch. The correct answer is complex. In the world of 27 A.D. with the religious formation and the spiritual identity taking shape within the heart of Jesus, the answer would be yes. His baptism represented a change in his life, an embrace of a unique call by his heavenly Father, and acceptance of the struggle that would come with that, and resignation that such might cost him his life. One can, I believe, safely assume that Jesus is committing himself to what he learned from John that final destiny and judgment were soon at hand.
In our times, though, the Holy Spirit has guided the Church through the teachings of the later evangelists, St. Paul, Church Councils, and the doctors of the Church toward a somewhat different understanding of Baptism in the light of hindsight, looking back upon the entire life, death, and resurrection of the Savior. St. Paul in particular points us to an understanding of the Sacrament as a dying and rising with Christ, and St. Luke among others has developed what we might call a "baptismal lifestyle" of how a family of believers would look and act. The Church has never lost sight of the fact that Baptism is indeed a washing of sin with an eye toward future judgment and consequences; at the same time, it has come to understand the act in a much more developed fashion that would've been available to John the Baptist.
Thus, the feast of the Baptism of the Lord is primarily a feast about the Savior and his identity. By setting this baptism in its proper historical context, we understand the nature and mission of Jesus with more clarity. The Baptism of Jesus is not pastorally nor doctrinally identical to the Baptism in the Catholic sacramental system, and it may be pedagogically confusing to speak of the two as if they were the same thing.
It would be interesting to learn how you have addressed this in your ministry this week and how the feast was celebrated in your parish and how the event of Jesus' baptism was preached and/or explained to your community.
For an interesting interview with Father Meier from St. Anthony Messenger, see here.