Gaudete Sunday is celebrated on December 17, 2017. It is the third Sunday of Advent in the liturgical calendar of the Western Church, including the Catholic Church, the Anglican Communion, the Lutheran Churches, and other mainline Protestant churches. It can fall on any date from 11 December to 17 December. The day takes its common name from the Latin word Gaudete ("Rejoice"), the first word of the introit of this day's Mass.
On Gaudete Sunday rose-coloured vestments may be worn instead of violet, (or in the Anglican tradition and some Lutheran traditions, Sarum blue) which is otherwise prescribed for every day in the season of Advent. This tradition, previously informally observed in the Anglican Church, was formally noted as an option in the Church of England in the Common Worship liturgical renewal.
In churches which have an Advent wreath, the rose coloured candle is lit in addition to two of the violet (or blue) coloured candles, which represent the first two Sundays of Advent. Despite the otherwise somber readings of the season of Advent, which has as a secondary theme the need for penitence, the readings on the third Sunday emphasize the joyous anticipation of the Lord's coming. (With material from: Wikipedia)