The third Sunday of Advent, also called Gaudete Sunday is named this from the first word of the Entrance Antiphon, (Gaudete in Latin, which means rejoice). Advent was, in its earliest record, was a 40 day period of fasting in preparation for Christmas, beginning on November12th.
In the ninth century, Advent was reduced to four weeks. Advent preserved most of the characteristics of a penitential seasons which made it a kind of counterpart to Lent, when on the fifth or Laetare Sunday, the organ and flowers were, permitted to be used. Rose-coloured vestments were also allowed instead of purple. All these distinguishing marks have continued in use today. Gaudete Sunday is further marked by a new Entrance Antiphon. The Church calls the faithful to worship and hail with joy “The Lord who is now nigh and close at hand.”
The joy of expectation is emphasized by the constant Alleluias, which occur throughout the entire season. … The spirit of the Liturgy all through Advent is one of expectation and preparation for the Christmas feast as well as for the second coming of Christ. The penitential exercises suitable to that spirit are suspended on Gaudete Sunday in order to symbolize that joy and gladness in the promised Redemption which should never be absent from the heart of the faithful (catholic.org, Catholic Online, 2019).