Indian summer (n.) Look up Indian summer at Dictionary.com
"spell of warm weather after the first frost," first recorded 1778, American English, perhaps so called because it was first noted in regions inhabited by Indians, or because the Indians first described it to the Europeans. No evidence connects it with the color of fall leaves or a season of Indian attacks on settlements. It is the American version of British All-Hallows summer, French été de la Saint-Martin (feast day Nov. 11), etc. Also colloquial was St. Luke's summer (or little summer), period of warm weather occurring about St. Luke's day (Oct. 18).