Indian Summer, popular expression for a period of mild, summerlike weather which occurs in the autumn, usually after the first frost. The origins of the name are obscure, but it was in use early in the 19th century in Canada and even earlier in the US. The Toronto Meteorological Observatory (est 1839) recorded the date of certain periodic events, such as Indian summer, until 1871. Almost invariably, its records show that this phenomenon occurred in late October or the first half of November and lasted from a few days to about a week. Although it cannot be precisely defined, Indian summer continues to arrive most years just as thoughts of the summer past are fading into thoughts of the winter to come.