The second week of Christmas marks the end of the Christmas season in the church calendar. The Feast of the Epiphany falls on the 12th day after Christmas (January 6th) and celebrates the visit of the three kings or wise men to the Christ Child, signifying the extension of salvation to the Gentiles.
The Christmas season officially ends the Feast of the Baptism of the Lord.
Traditionally, the Feast of the Baptism of the Lord was celebrated on January 13, the octave day of Epiphany. In the current liturgical calendar, the Baptism of the Lord is celebrated on the Sunday after January 6.