Beginning the Church’s liturgical year, Advent (from, “ad-venire” in Latin or “to come to”) is the season encompassing the four Sundays (and weekdays) leading up to the celebration of Christmas.
The Advent season is a time of preparation that directs our hearts and minds to Christ’s second coming at the end of time and also to the anniversary of the Lord’s birth on Christmas. The final days of Advent, from December 17 to December 24, focus particularly on our preparation for the celebrations of the Nativity of our Lord (Christmas).
Advent devotions, including the Advent wreath, remind us of the meaning of the season. The purple candles symbolize the prayer, penance, and preparatory sacrifices and good works undertaken at this time. The rose candle is lit on the third Sunday, Gaudete Sunday, when the priest also wears rose vestments at Mass; Gaudete Sunday is the Sunday of rejoicing, because the faithful have arrived at the midpoint of Advent, when their preparation is now half over and they are close to Christmas.
(Taken from United States Conference of Catholic Bishops website)