"Gaudete" comes from the Latin Antiphon, which begins, "Gaudete in Domino semper: iterum dico, gaudete.." [Rejoice in the Lord always, again I say, rejoice...]. On this day, rose-colored vestments are worn, and flowers may decorate the chancel of the church.
Angel in the folds of the Madonna's Robe
Basilica Santa Anastasia on the Palatine Hill
Father of our Lord Jesus Christ, ever faithful to your promises and ever close to your Church: the earth rejoices in hope of the Savior's coming and looks forward with longing to his return at the end of time. Prepare our hearts and remove the sadness that hinders us from feeling the joy and hope which his presence will bestow, for he is Lord for ever and ever.
The Opening Mass Prayer for the Third Sunday of Advent
From the Companion Missal