From the Latin term 'adventus', Advent was used in a secular context to denote the coming or arrival of the emperor.  In the Catholic Liturgy, Advent is a period beginning with the Sunday nearest to the feast of St. Andrew the Apostle (30 November) and embracing four Sundays. It marks the beginning of the ecclesiastical year in the West.  

Advent has a twofold character: as a season to prepare us for Christmas when Christ's first coming is remembered; and as a season when that remembrance directs our minds and hearts to await Christ's Second Coming at the end of time.  It is thus a period for devout and joyful expectation. During this period we are admonished to make our souls fitting abodes for the Redeemer's coming in Holy Communion and through grace thereby making ourselves ready for His final coming as judge, at death and the end of the world.

During Advent violet vestments are used (with rose as an option on the third) or "Gaudete" Sunday and the preaching of John the Baptist speaks of the penitential aspect inviting us to reform. 

The First Sunday of Advent is clearly centred on the Lord's Second Coming and the Preface used at Mass until 16 December emphasizes this theme.  The Gloria is omitted at the Advent Masses but for a somewhat different reason, as the official commentary on the revised Calendar notes: "So that on Christmas night the song of the angels may ring out anew in all its freshness."