Rejoice, the Lord is Near! Gaudete Sunday, Ember Days and O Antiphons
The rose-colored vestments for the Third Sunday of Advent or Gaudete Sunday are a signal for the transition into the second part of Advent, which is provides more intense preparation for the remaining days of Advent. Besides Gaudete Sunday, the Advent Ember Days and O Antiphons are the special treasures of the second phase of the Advent liturgy.
Source: catholicculture.org