According to Psychology Today, it is "entirely normal to feel a sense of sadness" after Christmas. They suggest that one way to cope with post-holiday blues is to realize that others feel the same way. Indeed, as the saying goes, "Misery loves company."
However, for Christians, Christmas is not yet over. According to the Church Calendar, Christmas is not just a single day, but a twelve-day season known as the Twelve Days of Christmas. During this time, we celebrate the birth of Christ and the miraculous event of the Incarnation. And after the twelve days, we have another holiday called Epiphany, where we commemorate the visit of the Magi and the manifestation of Christ to the Gentiles.
So, on this Third Day of Christmas, let us rejoice! Christ the Lord is born! There is still more celebration, more Christmas music to enjoy, and more Christmas happiness to share.