From Universal Norms onthe Liturgical Year and the General Roman Calendar:
39. 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.
40. 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.
41. The Sundays of this time of year are named the
First, Second, Third, and Fourth Sundays of Advent.
42. 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 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. 305 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.