Certain Sundays throughout the liturgical year have taken their names from the first word in Latin of the Introit - the entrance antiphon at Mass. Gaudete Sunday that we celebrate today is one of those Sundays. The Introit for this Sunday is taken from Saint Paul’s letter to the Philippians 4:4,5: "Gaudete in Domino semper" - "Rejoice in the Lord always". It is a joyous celebration! Although it takes place during the penitential season of Advent, Gaudete Sunday serves as a mid-point break from the austere practices to rejoice in the nearness of Christs’ coming.
Like Lent, Advent is a penitential season, so the priest normally wears purple vestments. But on Gaudete Sunday, the Church lightens the mood a little, and the priest may wear rose vestments. The change in color provides us with encouragement to continue our spiritual preparation - especially by prayer, acts of charity and fasting - for Christmas.
For this same reason the third candle of the Advent wreath lit on Gaudete Sunday, is traditionally rose-colored. It is a reminder that the Advent season is also a season of joy because our Salvation is already at hand. As Christmas draws near, the Church emphasizes the joy which should be filling our hearts over all that the birth of our Savior means for us and the entire world.