January 13th is the Feast of the Baptism of the Lord and the last day of the Christmas season. On the Feast of the Baptism of the Lord, the Church celebrates the Lord's second epiphany in which the Trinity is manifested. At the River Jordan, Jesus, although without sin, submits to the baptism of repentance administered by John the Baptist.

Readings:

Isaiah 42:1-4, 6-7
Psalm 29:1-4, 9-10
Acts 10:34-38
Matthew 3:13-17

Here are a few commentaries on these readings:

Pontifical Household preacher, Capuchin Father Raniero Cantalamessa (Courtesy of Zenit)

St. Charles Borromeo Lectionary Site (PDF)

St. Paul Center for Biblical Theology