Steve Wilkins writes in his helpful article:
Observing Christmas as a season helps us to move beyond the sentimentalism that has become so much a part of “Christmas” and commemorate the true significance of Jesus’ birth. It enables us to see that Jesus’ coming truly transforms all things. It marked the end of the old world (under the dominion of sin and death) and the beginning of the new. And it reminds us of our new identity and purpose. We are now children of the King and are called to rejoice and give thanks and show the world the new destiny that now has come in Him. To celebrate for twelve days (as opposed to one) enables us to realize afresh the significance of what happened in Bethlehem and it declares to the world the remarkable reality that Jesus has destroyed the works of the devil and established a kingdom that shall have no end.