Gaudete!

The third Sunday of Advent, so called from the first word of the Introit at Mass (Gaudete, i.e. Rejoice). The season of Advent originated as a fast of forty days in preparation for Christmas, commencing on the day after the feast of St. Martin (12 November), whence it was often called "St. Martin's Lent" - a name by which it was known as early as the fifth century. In the ninth century, the duration of Advent was reduced to four weeks, and by the twelfth century the fast had been replaced by simple abstinence. Notwithstanding all these modifications, however, Advent still preserved most of the characteristics of a penitential season, which made it a kind of counterpart to Lent, the middle (or third) Sunday corresponding with Laetare or Mid-Lent Sunday. On it, as on Laetare Sunday, rose-coloured vestments were allowed instead of purple (or black, as formerly).

(Adapted from The Catholic Encyclopedia).