Christmas is the liturgical season when Catholics rejoice over Christ having coming into the world to save man from sin and illuminate the path to eternal life. Christ came to acquire the graces men would need for salvation by His bodily death on the Cross and to physically commit His teachings to the Apostles. He further established a Church imbued with the Holy Spirit to mediate that grace and truth to all generations. In his book Life of Christ, Abp. Fulton Sheen spoke of Christ's twofold purpose in coming: "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 so they can joyfully receive Christ at Christmas in Holy Communion with a fresh appreciation for His gifts of truth and grace.
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" because as Matthew 1:21 says, "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, ending February 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 ends with the Baptism of the Lord, which directly follows the feast of Epiphany that's 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.