The following comes from Br. Eric Pohlman, a monk of St. John’s Abbey.
Below are the seven possible calendar scenarios for the Advent and Christmas seasons.
Transferring Epiphany to Sunday:
In scenario 2 Epiphany is already on a Sunday – no need to transfer.
In scenarios 3 through 6 Epiphany is moved “up” to take the place of the “Second Sunday after Christmas” (January 5, 4, 3, or 2).
In scenarios 1 and 7 the first available Sunday is after January 6 – Epiphany then displaces Baptism of the Lord.
In these years the Baptism of the Lord is instead celebrated the following day, Monday (January 8 or 9).
Other interesting things for this year (scenario 7):
We have the longest possible Advent – four full weeks – the first “O” Antiphon won’t be heard until the Vigil of the 4th Sunday.
Since Christmas Day and Mary, Mother of God (the Octave Day of Christmas) are on Sundays the feast of the Holy Family is on Friday, December 30.
The combined length of the two seasons is consistent in six of the seven scenarios – 43 days (6 weeks plus one day).
A day gained in Advent is lost in the week after Epiphany as you move left to right through scenarios 2 through 7.
Only in scenario 1 do the rules collude to reduce the combined length by a week – 36 days (5 weeks plus one day).
Five years out of seven we in the USA start Advent during the Thanksgiving holiday weekend.
The other two years it’s the feast of Christ the King.
The shortest Advent scenario is number one: twenty-two days.
The longest is scenario seven at twenty eight days.
The shortest Christmas season is scenario number one, which is:
fourteen days if Epiphany stays on January 6.
fifteen days if Epiphany is transferred to Sunday.
The longest Christmas season is scenario number two, at twenty days.
The shortest combined Advent and Christmas season is scenario number one which is:
thirty-six days if Epiphany remains on January 6.
thirty-seven days if Epiphany is transferred to Sunday.
The combined Advent and Christmas season is scenarios two-seven, which is
forty-three days if Epiphany remains on January 6.
forty-four days if Epiphany is transferred to Sunday: scenario seven.
Below is a chart showing this: