Q: Are Advent wreath candles to be lit for liturgies only (as are altar candles), or do they belong to the marking of time outside the ritual events as well?
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A: We have no governing rule on this. The blessing of an Advent wreath can be found in Chapter 47 of the Book of Blessings. It tells how to bless the wreath at Mass, and when to light candles on subsequent Sundays at Mass, so it expects that the candles will burn at least for Mass. But it is silent about other ritual events.
As this is a home custom that parish churches have adopted, it would be in keeping with its spirit to light the candles for any prayer during Advent. But we have no governing rule on this.