Christmas is a festival season on the church calendar that is sometimes called the Feast of the Incarnation. On the traditional church calendar it is a twelve day feast. But why would the incarnation of Jesus be so important that we would set aside almost two weeks to celebrate it.
The simple answer is that our hope and salvation is bound completely in the fact that the Son of God became a man. The book of Hebrews tells us that Jesus is a man like us in every way (Heb. 2:17). He faced every temptation, every limitation, everything human just like you and me. And because he is a human in every way, everything about our humanity is able to be redeemed.
As the early church defended the humanity of Jesus, the principle that they formulated was 'whatever was not assumed in the incarnation was not healed in the redemption.' And whatever was assumed by Jesus in the incarnation is being healed in our redemption. So the fact that Jesus became a person like us in every way means that in our redemption that everything human about us is being restored.
So even though sin has gotten into everything and into everyone, God's son became completely human so that we could be completely restored. “Therefore, in all things He had to be made like His brethren, that He might be a merciful and faithful High Priest in things pertaining to God, to make propitiation for the sins of the people” (Hebrews 2:17).
You could spend lifetimes contemplating this mystery, and you will spend an eternity living out the consequences of it.