Indian summer is an informal expression given to a period of sunny, warm weather in autumn in the northern hemisphere, typically in late October or early November, after the leaves have turned but before the first snowfall.
The origins of the term Indian Summer are most commonly thought to have derived from the timing of Summer in India to correlate with good weather in Autumn in the Western world.
In former times in Europe, Indian summer was called Saint Martin’s Summer, referring to St. Martin’s day, November 11, when it was supposed to end.
In Bulgaria, the phenomenon is sometimes called Gypsy Summer and in some places Gypsy Christmas and refers to unseasonably warm weather in late fall, or a warm spell in between cold periods.
In Sweden it is called brittsommar, which is derived from Birgitta and Britta, who have their name day in the Swedish calendar on October 7. That is when Britt Mass, an official fall open-air market, was held.
In Germany and Austria it is called Altweibersommer (Old Ladies Summer) because the many white spider silks seen at this time of the year have been associated with the norns of Norse folklore or medieval witches.
There are around 43 different theories concerning the origin of the term.