What are these and how are they used?
They are called vigil lights or votive lights, and they are an outward sign of your inner faith in the power of prayer, as well as a reminder that prayers continue to be offered up even after we walk out of the church building. The lit candle is also a reminder that Christ is the Light of the World, and the rising smoke is a symbol of prayer.
Generally, you light the candle and say a prayer for your intention (in either order). You may choose to leave a donation to help offset the cost of replacing the candle, but it is not required.
“Vigil Lights (from the Latin vigilia, which means “waiting” or “watching”) are traditionally accompanied by prayers of attention or waiting. Another common type of candle offering is the votive light. Such an offering is indicative of seeking some favor from the Lord or the saint before which the votive is placed.” (acatholiclife.blogspot.com/2006/03/why-do-catholics-light-prayer-candles.html)
*Also, I see a donation box. Is it okay to simply add money to my tithing contribution to cover the costs of any candles I light instead of paying there (I’m always afraid that it might give the impression I am paying for the prayer, instead of for the candle I’ve used up). *
If your intent is to pay for the candle, then why not put the offering where it’s most likely to be used just for that purpose? A donation is not required, and it wouldn’t be wrong to put it in the regular offering, but I really don’t think you need to worry about how putting a donation in a donation box might “appear” to others.
Most Catholics who would be around before/after Mass long enough to see you light the candle and put money in the box probably already understand what the votive donation is about. Most people who are in the church are going to be focused on their own prayers, anyway, not on what that dollar you put in the donation box is about. If you’re worried you’ll give your children that impression, then simply explain it to them.
When can I light these and what are the reasons to do so?
**Anytime the church is open **(except when it would be disruptive or distracting for others–for instance, it’s probably not a good idea to light the ones right up front during Mass), and for any prayer intention. You don’t even have to be Catholic or a member of the parish to light a candle for a prayer intention. You (and others) may also choose to pray for the intentions represented by the other lit candles (I try to remember to do this when I arrive early at Mass).
*When I light one, do I leave it burning or do I blow it out? *
Most people leave them burning as a sign of their intent to continue praying, as well as a reminder that the saints continually pray for us, and so that others may pray for those intentions, as I mentioned. The candles will go out on their own when they burn all the way down. If you do choose to blow yours out, be very careful not to blow out any others.
*Any other etiquette rules that I should understand? * As mentioned, don’t blow out any lit candles that are not your own–remember to be careful of this when blowing out the match you used. If you do accidentally blow one out, re-light it. But you probably could have figured those out yourself. Other than that, I can’t think of anything… don’t play with the matches?