The fourth and last sunday of the advent season is on December 20, 2015. The day is called the Fourth Sunday of Advent. The fourth candle is lit on this day along with the first three. It is the Angel's candle, reminding of the heavenly hosts that proclaimed Christ's arrival with "Behold, I bring unto you good tiding of great joy!"
The theme of readings and teachings during Advent is often to prepare for the Second Coming while commemorating the First Coming of Christ at Christmas. With the view of directing the thoughts of Christians to the first coming of Jesus Christ as savior and to his second coming as judge, special readings are prescribed for each of the four Sundays in Advent.
In recent times the most common observance of Advent outside church circles has been the keeping of an advent calendar or advent candle, with one door being opened in the calendar, or one section of the candle being burned, on each day in December leading up to Christmas Eve. The keeping of an advent wreath is also a common practice, with four or five candles extending from the wreath. The colours associated with the candles are variously interpreted. (With material from: Wikipedia)