https://infogalactic.com/info/Epiphany_season

I guess there are some modern shakeups with this in the Vatican 2 era, some of us are following the old tradition here:

Until 1955, the feast of the Epiphany in the Roman Rite of the Catholic Church had an octave, and was thus celebrated from the vigil Mass on 5 January until 13 January. After Pope Pius XII removed this octave, the liturgy of those days continued to use the same texts as previously, thus giving to the period until the Feast of the Baptism of the Lord, which replaced the Octave Day of the Epiphany, a special character

This defined the season of Christmas as extending from First Vespers of Christmas to 13 January inclusive, and divided this season into Christmastide, ending just before First Vespers of the Epiphany, and Epiphany ... from then until 13 January inclusive (one week).[11]

So Christmastide (the time after Christmas) was either defined as ending on January 5th (Epiphany), January 13th (Octave Day of Epiphany), or February 2nd (Candlemas): https://infogalactic.com/info/Christmastide

By the letter of the law, those (sedevacantists) who follow Pius XII as the last pope would then probably observe the abolished Octave of the Epiphany, but a perhaps more common approach by the spirit of the law has been to reject these changes (like the Easter week changes) as being leaning in a destructive modernist direction.