Advent, which begins the liturgical year of the Catholic Church, is  the season of preparation for celebrating Christ's Birth as well as the prayerful, spiritual preparation for the second and final coming of the Lord.
The emphasis in the Mass and the daily prayers of this season is on the threefold coming of Christ:

The prophecies of His Incarnation and Birth
His coming into our lives through grace and the sacraments, especially the Sacrament of Holy Communion
And, His Second Coming at the end of time

Advent is a period of joyful expectation but also of penance, as indicated by the liturgical color of the season— purple.

The Holy Father's Intentions for the Month of December 2018

Evangelization:
In the Service of the Transmission of Faith: That people, who are involved in the service and transmission of faith, may find, in their dialogue with culture, a language suited to the conditions of the present time. 

Feasts for December

The feasts on the General Roman Calendar celebrated during the month of December are:

December 2 - First Sunday of Advent, Sunday
December 3 - Francis Xavier, Memorial
December 6 - Nicholas, Optional Memorial
December 8 - Immaculate Conception of the Blessed Virgin Mary, Patronal Feastday of the United States of America , Solemnity
December 9 - Second Sunday of Advent, Sunday
December 11 - Damasus I, Optional Memorial
December 12 - Our Lady of Guadalupe (USA), Feast
December 13 - Lucy, Memorial
December 14 - John of the Cross, Memorial
December 16 - Third Sunday of Advent, Sunday
December 21 - Peter Canisius, Optional Memorial
December 23 - Fourth Sunday of Advent, Sunday
December 25 - Christmas, Solemnity
December 26 - Stephen, Feast
December 27 - John, Feast
December 28 - Holy Innocents, Feast
December 29 - Thomas Becket, Optional Memorial
December 30 - Holy Family, Feast
December 31 - Sylvester I, Optional Memorial