Indian summer (n.)

"Frost's departure", meaning a period of warm, dry, and hazy weather, occurs after the first frost of the year. This phenomenon usually takes place from mid-September to nearly December, depending on the location. The term originated in 1774 in North American English (also used in eastern Canada), perhaps because it was first observed in regions still inhabited by Native Americans or because the Indigenous people first reported it to the Europeans in the upper Mississippi valley to the west of the Appalachians. There is no evidence to support a connection between the term and the color of fall leaves or a season of renewed Native American attacks on settlements due to renewed warm weather (a widespread explanation dating back to at least the 1820s).

It is the American version of the British All-Hallows summer, the French été de la Saint-Martin (feast day Nov. 11), etc. Also colloquial were St. Luke's summer (or little summer), a period of warm weather occurring around St. Luke's day (Oct. 18). An older and simpler name for it was autumn-spring (1630s).