There is a term in the Church, called “Apodosis, or Leave-taking”, which represents the end of a particular festal period. For example, the Apodosis of Pascha is the day before Ascension. Just as with Pascha we stop greeting each other with, “Christ is Risen” after the apodosis, today (The Apodosis of Christmas) will be the final day we greet each other with, “Christ is Born!” Tomorrow begins another festal period for the Feast of the Circumcision of Our Lord on the Eighth Day.
So why does the Church speak about the Twelve Days of Christmas if December 31st is the Leave-Taking? Both feasts, Christmas and Circumcision, lead up to Theophany and the Baptism of Our Lord. In fact, in the earliest centuries, Theophany was the main feast with very little emphasis on the Nativity narratives. The Church began placing emphasis on Christmas only in the Fourth Century, not that 1600 years doesn’t make it an ancient feast.
The Church celebrates Theophany as the appearing of the Holy Trinity in creation, while Christmas was the birth of the eternal Son of God. Once the Church extended the celebration to include the Nativity, it was natural to “link Christmas to Theophany,” so much that the hymns for the Royal Hours of Christmas actually include the words, “We worship Your Nativity O Christ, show us also you Divine Theophany.” The hymn is sung in the same manner as Holy and Great Friday when the Church prayers, “We worship your Passion O Christ, show us also Your Holy Resurrection.” Just as the Passion LEADS to the Resurrection, the Birth LEADS to Theophany. Thus.....the Twelve Days of Christmas.