Sometimes the candles or candelabras are lit before the guests arrives. Other times the ushers light them as part of the prelude, or as part of the wedding ceremony.
If you choose to have a Unity Candle, the parents or mothers of the Bride and Groom, may each light one of the two candles from which later the couple will light their Unity Candle. Typically, the mothers or parents would do this just before they are seated during the processional.
Be sure to test light all of the candles, even the Unity Candle, during the rehearsal, or at some time prior to the actual wedding ceremony. Test lighting is done to ensure that the candles will stay lit and not be extinguished by an overhead air duct, draft or fan.
Often times this simple step of candle lighting is overlooked in the wedding planning. Be sure to decide who will light the candles, when they will be lit, and how they will be lit. Find out if the church provides a candle lighter/snuffer or if these items will need to be rented.