The liturgical year begins with Advent–this was true prior to Vatican II, of course. The sense of Advent prior to the council was a bit more penitential, hence this witness of Pope Pius:
154. In the period of Advent, for instance, the Church arouses in us the consciousness of the sins we have had the misfortune to commit, and urges us, by restraining our desires and practicing voluntary mortification of the body, to recollect ourselves in meditation, and experience a longing desire to return to God who alone can free us by His grace from the stain of sin and from its evil consequences.
This reflection on the Nativity is interesting:
155. With the coming of the birthday of the Redeemer, she would bring us to the cave of Bethlehem and there teach that we must be born again and undergo a complete reformation; that will only happen when we are intimately and vitally united to the Word of God made (flesh) and participate in His divine nature, to which we have been elevated.
Christmas as a time to reflect on the Christian experience on being born again. And the latter part of the Christmas season leads us to a deeper sense of gratitude:
156. At the solemnity of the Epiphany, in putting before us the call of the Gentiles to the Christian faith, she wishes us daily to give thanks to the Lord for such a blessing; she wishes us to seek with lively faith the living and true God, to penetrate deeply and religiously the things of heaven, to love silence and meditation in order to perceive and grasp more easily heavenly gifts.