Today, I found that the radio stations had gone back to playing their usual fare, the stores were removing their holiday displays, and people looked at me funny when I wished them a merry Christmas. But today is Christmas! You may be thinking, "Nick, check your calendar, buddy, it's the 26th," to which I would respond, "Check your liturgical calendar, friend, it's Christmas today!" See, there is in the Church an octave for Christmas, meaning that this feast, like Easter, is celebrated for not just one day, but for eight days--and each day is just as much that feast as any other. The Liturgy of the Hours uses many of the same prayers for these eight days; the Mass uses the same collects and prefaces; as far as the liturgy is concerned, it's all one. So, in a sense, today is Christmas just as much as yesterday!
(Now, today is also the feast of St. Stephen, deacon and martyr, also called the Proto-Martyr because he is the first Christian to have been killed for his faith, but that doesn't stop it still being Christmas.)
The octave ends on January 1, which is itself a great holy day: the Solemnity of Mary, Mother of God, formerly known as the "Feast of the Purification of Mary," referring to the ritual for women post-childbirth which Mary would have undergone on this day. 5 days after that is the Feast of the Epiphany, in which we celebrate the visit of the Magi, which represents Christ's manifestation (or "epiphany") to the whole world. These 12 days from Christmas to Epiphany are the Christmas season. So, THAT'S where the "twelve days of Christmas" come from!
In previous times, though, the Christmas season was made to last 40 days, up until the Feast of the Presentation of Jesus, commemorating his circumcision and thus his entrance into the covenant of Abraham, which was to be fulfilled in its totality by him. 40 days is a nice biblical number and corresponds well with the 40 days of Lent and the 40 days between Easter and the Ascension.
All of this is to help answer a very important question: how long can my Christmas lights stay up? I'd say you've got a good case for leaving them up all the way to February 2. But if they're still up at, say, Pentecost, you're pushing it. But, either way: Merry Christmas to you all!