The Feast of the Baptism of the Lord is on a Monday in 2017? I thought it was always the Sunday after Epiphany?
Okay, just checked my Missal:
Where the Solemnity of the Epiphany is transferred to Sunday, if this Sunday occurs on January 7 or 8, the Feast of the Baptism of the Lord is celebrated on the following Monday.
So Epiphany is normally January 6th. In many locales (like the United States), it gets transferred to the Sunday after January 1st. The Baptism of the Lord is the Sunday after Epiphany—except when Epiphany falls on January 7th or 8th.
And I was looking forward to an extended Christmas season.
To answer the OP, the Christmas Season starts on Christmas day and ends with the feast of the Baptism of the Lord. The day after the Baptism of the Lord is when Ordinary Time starts and the priests go back to wearing green vestments for Mass.