Q: Why did Baptism of the Lord fall on a Monday this year and not a Sunday?
A: Because of Epiphany. The entire Catholic world is expected to go to mass for the Epiphany, so if January 6 is not a holyday of obligation in your country, and it’s not in the US, then the epiphany gets transferred to a Sunday around that date. The Baptism of the Lord follows on the next Sunday.
But if Epiphany falls on a Sunday AFTER January 6, it removes the possibility of celebrating the Baptism of the Lord on a Sunday because the liturgical books don’t have enough masses to extend Christmas Time beyond January 13.
When our calendar went into effect after Vatican II, it ELIMINATED the feast of the Baptism of the Lord whenever Epiphany fell on January 7 or 8. But Pope Paul VI permitted us to celebrate it on a Monday. So, the problem is that we’ve run out of Sundays. As a remedy, Christmas Time is not shortened by a week, but extended by one day.