Out of the almost 8 years practicing the Craft, I have only cast the circle a handful of times. I do not often cast the circle, I will say however, that it is at the discretion of the Wiccan. If one feels they must cast the circle at a Sabbat, then so be it. If one feels they must cast the circle during mediation, than so be it. I personally do not think the circle needs cast for everything, the main thing I think it should be cast for is Sabbats. Unless the Wiccan is trying to bolster the atmosphere, or for extra protection, I don't think the circle need be cast.
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Aphren