For Catholics, Christmastide isn't an elongated birthday party for baby Jesus. It's a liturgical season when they rejoice over Christ having come to save man from sin.
Christ came to die on earth so that man could live in Heaven. As Abp. Fulton Sheen noted in his book Life of Christ, the baby Jesus was born "to die."
Sheen spoke of the babe's twofold role as savior and teacher: "He presented Himself as a Savior rather than merely as a Teacher. It meant nothing to teach men to be good unless He also gave them the power to be good."
Advent prepares Catholics to rejoice during Christmastide in Christ's salvific mission by fruitful meditation on Christ's three comings: as humble Savior, as future judge and as silent Eucharistic Lord. Catholics conduct spiritual house cleaning during Advent to spiritually prepare them for Christmastide. A fruitful Advent redounds to a joyful Christmas.
The Christmas octave begins with the vigil Mass on Christmas Eve and extends to the feast of Mary, Mother of God on New Year's Day. On that day, as recorded in Luke 2:21, the eight-day-old Christ was circumcised in the Temple and given the name "Jesus."
Matthew 1:21 further elaborates, "He shall save people from their sins." During the octave of Christmas, the Church witnesses to Christ's work of salvation by celebrating the deaths of St. Stephen, the Holy Innocents and St. Thomas à Becket.
Christmastide traditionally lasts 40 days after Christmas Eve. It ends Feb. 2 on Candlemas Day with simultaneous feasts: the Purification of the Blessed Virgin Mary and the Presentation of her divine Child Jesus in the Temple. In the Novus Ordo rite, however, the liturgical season concludes with the Baptism of the Lord, which directly follows the feast of Epiphany on the twelfth day of Christmas.
See how this great feast is attacked in Church Militant's Premium show Mic'd Up—The Assault on Christmas.