The third Sunday of Advent is also known as Gaudete Sunday. It is so named after the first word in the Introit: Gaudete (Rejoice!).
The Introit from the usus antiquior Romanus
(ancient Roman use):
Rejoice in the Lord always: again I say rejoice. Let your modesty be known to all men: for the Lord is nigh. Be nothing solicitous: but in everything by prayer let your petitions be made known to God. Ps. 84:2 Lord, Thou has blessed Thy Land: Thou hast turned away the captivity of Jacob.
Gaudete in Domino semper: iterum dico, gaudete. Modestia vestra nota sit omnibus hominibus: Dominus enim prope est. Nihil solliciti sitis: sed in omni oratione petitiones vestrae innotescant apud Deum. Ps. 84:2 Benedixisti, Domini, terram tuam: avertisti captivitatem Jacob.
Advent still has most of the characteristics of the penitential season, which make it a kind of Lent, the middle (or third) Sunday corresponding with Laetare or Mid-Lent Sunday. On it, as on Laetare Sunday, organ and flowers, forbidden during the rest of the season, are allowed to be used; rose-colored vestments are allowed instead of purple.
This distinguishing liturgy is a present discipline of the Church. Gaudete Sunday, therefore, makes a break, like Laetare Sunday, in the midst of our penitential rites, and signifies the nearness of the Lord's coming.
In both the Divine Office and Mass throughout Advent, there is reference made to our Lord's second coming, and this is emphasized on the third Sunday by the additional signs of gladness and joy permitted on that day.
I already decorated our home altar with a rose cloth and Father had on rose colored vestments and a rose chalice veil at Mass. :)