The ceremony is simple. It starts at the evening meal on the Saturday before the first Sunday in Advent with the blessing of the wreath.
(The head of the household is the one designated to say the prayers following which various members of his family light the candles. If the group is not a family then a leader may be selected to say the prayers and others appointed to light the candles.)
For blessing the wreath the following prayer is suggested:
Father: O God, by whose word all things are sanctified, pour forth thy blessing upon this wreath, and grant that we who use it may prepare our hearts for the coming of Christ and may receive from Thee abundant graces. Through Christ our Lord. Amen.
Then follows the prayer which is said before the evening meal each night of the first week in Advent.
The First Week
Father: O Lord, stir up Thy might, we beg Thee, and come that by Thy protection we may deserve to be rescued from the threatening dangers of our sins and saved by Thy deliverance. Through Christ our Lord. Amen.
On this night the first purple candle is lighted by the youngest child of the household and is left burning during the meal. Each night of that week the first candle is lighted before the meal and extinguished after the meal.
The Second Week
The following prayer is said before the evening meal and each night of the second week in Advent.
Father: O Lord, stir up our hearts that we may prepare for Thy only begotten Son that through His coming we may be made worthy to serve Thee with pure minds through Christ our Lord. Amen.
Then the eldest child lights not only the first but a second purple candle. Both candles burn during the evening meal as before.
The two candles are extinguished immediately following the meal.
The Third Week
The following prayer is said before the evening meal and each night of the third week in Advent.
The joyful Sunday in Advent (known as Gaudete) is represented by rose (or pink) instead of the penitential purple color. Each night during the third week the mother of the family lights the pink as well as the two previously burned purple candles after the following prayer has been said.
Father: O Lord, we beg Thee incline Thy ear to our prayers and enlighten the darkness of our minds by the grace of Thy visitation. Through Christ our Lord. Amen.
The three candles are extinguished immediately following the meal.
The Fourth Week
The father of the household lights all four candles in proper sequence during the fourth week. The following prayer is said before the evening meal and each night of the fourth week in Advent.
Father: O Lord, stir up Thy power, we pray Thee, and come; and with great might help us, that with the help of Thy Grace, Thy merciful forgiveness may hasten what our sins impede. Through Christ our Lord. Amen.
The four candles are extinguished immediately following the meal.
Candles can be replaced as necessary during a particular Advent Season without reblessing the wreath.
For the Christmas Season which follows immediately after Advent, candles and ribbons can be changed to white. If you wish, the wreath itself can be freshened with new greens and decorated festively for use during the holiday period. Lighting all four white candles to burn during the principal meal each day of the Christmas Season is a customary and appropriate practice.
Prayer Source: Advent Wreath Prayers catholicculture.org