Lighting the unity candle, though not part of the Catholic rite of marriage, is common in Catholic weddings. A unity candle is not required, and in some churches, it is not allowed.
Lighting candles, however, does have great significance in the Catholic Church. Our most important candle is the Paschal or Easter Candle, lit at the Easter Vigil. From this candle, all the candles given at infant and adult baptisms are lit. We honor the Paschal Candle because it represents Christ, the Light.
One reason the church’s marriage liturgy does not include lighting the unity candle may be because its popular meaning—two lives becoming one—is already profoundly signified through the couple’s exchange of vows and rings and in the nuptial blessing.
Some parishes allow the unity candle practice because of its personal meaning to the couple. If permitted, the lighting should be done outside The Order of Celebrating Matrimony itself. For example, an appropriate time might be at the close of the liturgy, just before the final blessing and recessional.