This, not Advent, is the true Christmas Season. As most people in secular or Protestantized countries are putting away "Christmas-y" things, and as shopping malls stop blaring "Here Comes Santa Claus," Catholics are just getting started. The cleaning and baking during penitential Advent pays off now, and the feasting and caroling begin!
The entire Christmas Cycle is a crescendo of Christ's manifesting Himself as God and King -- to the shepherds, to the Magi, at His Baptism, to Simeon and the prophetess, Anna (Luke 2). The days from the Feast of the Nativity to the Epiphany are known as "The Twelve Days of Christmas," with Christmas itself being the first day, and Twelfthnight -- 5 January -- being the last of the twelve days. Christmastide liturgically ends on 13 January, the Octave of the Epiphany and the Baptism of Christ (at which time the season of Time After Epiphany begins). But Christmas doesn't end spiritually -- i.e., the celebration of the events of Christ's life as a child don't end, and the great Christmas Cycle doesn't end -- until Candlemas on 2 February and the beginning of the Season of Septuagesima.
In this way, just as From Ash Wednesday on, we commemorate Christ in the desert for forty days, and just as after Easter we celebrate for forty days until the Ascension, after Christmas we celebrate the Child Jesus for forty days -- all through the season of Time After Epiphany -- until Candlemas. The schema of those Christ Child celebrations looks like this:
Christ is born
Feast of the Holy Innocents
Herod slaughters the baby boys in order to kill the Christ Child
The Circumcision (the Octave of Christmas)
Jesus follows the Law
Feast of the Holy Name of Jesus
After He is circumcised, He is named and becomes a part of the Holy Family
The Twelve Days of Christmas as a Feast come to an end
Feast of the Epiphany
Jesus reveals His divinity to the three Magi, and during His Baptism, and at the wedding at Cana
Baptism of Our Lord/Octave of the Epiphany
Christmas liturgically ends with the Octave of the Epiphany.
Feast of the Holy Family
Jesus condescends to be subject to His parents
Feast of the Purification (Candlemas)
40 days after giving birth, Mary goes to the Temple to be "purified" and to "redeem" Jesus per the Old Testament Law of the firstborn. Christmas truly ends as a Season with Candlemas and the beginning of Septuagesima.