While you're supposed to have a "killing frost" (as in, cold enough to kill plant life) and a long stretch of cold weather first to have the subsequent warm period count as an Indian Summer, around here, our definitions are less strict. I'd have to say this past week's weather qualifies.
As to the term's origin, it seems to have begun in colonial America, but there are several different versions of how the term came about; none of which are concrete. Some speculate it would allow the native Indians bonus time to harvest before the coming winter. Some say it was the Indian's hunting season.
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