Traditionally, Advent wreaths are constructed of a circle of evergreen branches into which four candles are inserted, representing the four weeks of Advent. Ideally, three candles are purple and one is rose, but white candles also can be used.
The purple candles symbolize the prayer, penance and preparatory sacrifices and goods works undertaken at this time. The rose candle is lit on the third Sunday, Gaudete Sunday, when the priest also wears rose vestments at Mass. Gaudete Sunday is the Sunday of rejoicing because the faithful have arrived at the midpoint of Advent when their preparation is now half over and they are close to Christmas.
The progressive lighting of the candles symbolizes the expectation and hope surrounding our Lord’s first coming into the world and the anticipation of his second coming to judge the living and the dead.
. U.S. Conference of Catholic Bishops - www.usccb.org.