It is that time of the year again when the calendar moves to September and by at least one metric, it is now autumn. Some say that it is still summer, but by calling it autumn, one can pronounce any sunny day as a harbinger of an Indian summer (are we allowed to call it that anymore?)
Here is the Wiki about the origin of the phrase:
Later research showed that the earliest known reference to Indian summer in its current sense occurs in an essay written in the United States around 1778 by J. Hector St. John de Crevecœur, describing the character of autumn and implying the common usage of the expression
Great rains at last replenish the springs, the brooks, the swamp and impregnate the earth. Then a severe frost succeeds which prepares it to receive the voluminous coat of snow which is soon to follow; though it is often preceded by a short interval of smoke and mildness, called the Indian Summer. This is in general the invariable rule: winter is not said properly to begin until those few moderate days & the raising of the water has announced it to Man.
So after the first frost, if we have a few mild days (and statistics should predict this reversion to the norm), then we will have our ‘Indian Summer’