As the old Catholic Encyclopedia (click on this post's title) and other sources explain, Gaudete Sunday takes its name from the first word of the verse in St. Paul's Epistle to the Philippians: Gaudete in Domino semper: Iterum dico, gaudete. (Rejoice in the Lord always: Again I say, rejoice.)
Liturgy professor Father Edward McNamara wrote in 2004:
"...From all this it is clear that the custom of using rose-colored vestments on Gaudete and Laetare Sundays is to be maintained whenever possible...Rose, which has never enjoyed frequent use, serves as a reminder, by using an unusual color, that we are halfway through a penitential season."
If the priest in your parish follows tradition and wears pink vestments this Sunday, be sure to thank him -- and please let ole Q here know he wore them.
Gaudete in Domino semper!