I get asked this question a lot and I know if you do a search on the internet, you can come up with a variety of definitions of what actually is "Indian Summer."  But the version I learned 30-some years ago seems reasonable and no one really can discount it.  The explanation goes as follows.

In the old Indian tribes, back in the day, it was up to the braves to do enough hunting and fishing during the summer to lay in an adequate store of food for the winter.  And most of them did, but there would always be one or two lazy ones who just didn't get it done.  They would sit around with their feet propped up, thinking they could get the extra hunting and fishing done later.  And then, it appeared they had waited too long.  A widespread killing frost would occur, ending the growing season...perhaps even a few snowflakes would be sighted.  It looked like winter was here.  But then would come a spell or two of sunny, mild days and clear, crisp nights.  It served as a reprieve for those who hadn't been doing what they should have been doing during the warm weather months.  So, the lazy Indians would get out there and fish and hunt like crazy in order to play "catch up."  This spell of weather became known as "summer for the lazy Indian"...which, over time, became shortened to simply "Indian Summer."

 Many years ago, I worked in Syracuse, New York, surrounded by various Indian tribes...most notably the Onondagas.  They agree with this description of Indian Summer, although some of them disagree with each other over whether a killing frost is all that's necessary or if you need to have an actual snowfall before it can happen.  Some tribesmen even suggested you had to have enough snow to track a deer before you could have real Indian Summer.  But the most widespread, accepted definition of Indian Summer is the one I have given you.

In some years, you might have just one or two spells of Indian Summer.  In other years (El Nino winters are an example), you can have Indian Summer episodes in November, December, or even early January.  We're heading into an El Nino now, as a matter of fact, so we'll see how many spells of this great weather we can squeeze in.  Enjoy!