Each year, many wonder why one of the candles on their Advent Wreath is a different color, light purple, rose-colored, or pink, instead of full-on purple or blue.
In some circles, this candle represents the Third Sunday of Advent, also known as Gaudete Sunday. Gaudate is latin for "rejoice" the first word of the Philippians epistle reading used on this day in the Middle Ages. Even today, the readings reflect this idea of rejoicing.
In the Middle Ages, it was thought this Sunday was an appropriate time to lessen the penitential nature of Advent. Rose pink became the color because it was a fading of the colors already used (see Faith Fact on Advent colors).
There are two reasons not to use rose pink vestments this Sunday. One is that it divides the season of Advent. The other is that Sunday is always a feast, no matter the season. There's no reason to draw attention to the less penitent nature of any Sunday because it is always there. Sunday is the day where we remember the death and resurrection of our Lord Jesus Christ. We take part in that death and resurrection too.
Every Sunday is a day to rejoice. There's no need to divide Advent up to prove it.
Source:
Bower, James. The Companion to the Book of Common Worship. Louisville: Geneva Press, 2003, pg.100.