The Christmas Octave

Another element to throw into the mix is that the “octave” of Christmas ends on the eighth day after Christmas, which is January 1st, the Solemnity of Mary, the Mother of God. Because Christmas is a Solemnity, the highest ranking of all feast days, it is not celebrated for just 24 hours; it is given the honor of eight days (octave) of celebration which is a custom that traces its roots to Old Testament feasts.

The end of the octave is the end of the Christmas feast proper, after which begins the longer Christmas season that extends either to the Baptism of the Lord for the Ordinary Form of the Mass (usually the Sunday after Epiphany) or the Purification of Mary for the Extraordinary Form of the Mass (February 2 – Candlemas). However this understanding of a proper Christmas ‘feast’ in some Eastern traditions is also associated with the 12 Days of Christmas which culminates on Epiphany.

From www.catholiccompany.com