On each of the four Sundays before Christmas a candle is lit with the one in the centre being lit on Christmas Day.  Traditionally there are four purple candles representing Hope, Peace, Joy and Love with the white Christ candle in the centre representing the coming of the Light of Christ into the world.

On the First Sunday of Advent.
A candle is burning, a flame warm and bright;
A candle of Hope in December’s dark night.
While angels sing blessings from heaven’s starry sky
Our hearts we prepare now, for Jesus is nigh.

On the Second Sunday of Advent........

(This is in a Church of Scotland but judging by the replies, perhaps purple is not common in Scotland. Purple was used in my last churches in England but before we used red or white candles in Scotland)