Universal Norms on the Liturgical Year and the General Roman Calendar of the
Advent has a twofold character, for it is a time of preparation for the Solemnities of Christmas, in which the First Coming of the Son of God to humanity is remembered, and likewise a time when, by remembrance of this, minds and hearts are led to look forward to Christ's Second Coming at the end of time. For these two reasons, Advent is a period of devout and expectant delight.
Advent begins with First Vespers ( Evening Prayer I) of the Sunday that falls on or closest to November 30 and it ends before First Vespers (Evening Prayer I) of Christmas.
The Sundays of this time of year are named the First, Second, Third, and Fourth Sundays of Advent.
The weekdays from December 17 up to and including December 24 are ordered in a more direct way to preparing for the Nativity of the Lord.
The liturgical color for Advent is purple, just like Lent - as both are seasons that prepare us for great feast days. Also Advent (like Lent) includes an element of penance in the sense of preparing, quieting and disciplining our hearts for the full joy of Christmas.
This penitential dimension is expressed through the color purple, but also through the restrained manner of decorating the church and altar: "During Advent the floral decoration of the altar should be marked by a moderation suited to the character of this time of year, without expressing in anticipation of the full joy of the Nativity of the Lord...[also] the use of the organ and other musical instruments should be marked by a [similar] moderation..."(GIRM n. and n. 313)
The third Sunday of Advent is called "Gaudete" Sunday (coming from the first word of the Latin Entrance Antiphon for this day, meaning "Rejoice") and the liturgical color may be rose instead of purple. This is the Church's way of further heightening our expectation as we draw ever nearer the Solemnity of Christmas.