Today is the last day of the Christmas season, but just what feast you are celebrating today will depend on which calendar you are following!
Once upon a time of course it was the Octave day of the Epiphany, aka the Commemoration of the Baptism of Our Lord. The octave was abolished by Pope Pius XII, but the 1962 Benedictine calendar still retains the Office of the traditional feast, as does the Novus Ordo calendar.
In the 1962 Roman calendar, though, the Sunday after the Epiphany, by order of Pope Benedict XV, is the Feast of the Holy Family.
I guess it isn't an entirely odd way to end Christmastide, but personally I think the three theophanies celebrated in the feast of the Epiphany (the Adoration of the Magi; the miracle at the wedding at Cana; and the Baptism of Our Lord) make a better segue into 'time throughout the year', or the Sundays after Epiphany, and as such, justify the restoration of the Octave - and celebration of the Octave Day itself, Sunday or not!
So finally, I've actually found a revision of the calendar that I actually agree with...