Q: If we have relics exposed for veneration on the feast of a saint, when do they get incensed?
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A: Here’s a quote from GIRM 277: “Two swings of the thurible are used to incense relics and images of the Saints exposed for public veneration; this should be done, however, only at the beginning of the celebration, following the incensation of the altar.”
These should be arranged on a table or stand, not on top of the altar. See the Directory on Popular Piety and the Liturgy #237.