Here's the definition given:
orig. N. Amer.
1. A period of unusually calm dry warm weather, often accompanied by a hazy atmosphere, occurring in late autumn in the northern United States and Canada; a similar period of unseasonably warm autumnal weather elsewhere.
However, the weather of late has been neither calm nor dry, and warm and hazy are understatements. Nor is it late autumn -- summer is still in full swing for almost another full month.
Perhaps, the OED had in mind the second definition?
2. fig. A late period in the life of a person or in the existence of a nation, culture, etc., characterized by calm, happiness, or achievement.
Don't think I'll go there.