Indian summer
1 a period of dry, warm weather occurring in late autumn.
2 a tranquil or productive period in someone's later years.
☞ The expression originated in the USA, where it seems to have alluded to the relatively warm autumns experienced in the parts of the country mainly inhabited by Native Americans, in contrast with the cooler weather of the east coast.
❷1930Vita Sackville-WestThe Edwardians Meanwhile she was quite content that Sebastian should become tanned in the rays of Sylvia's Indian summer.