The previous few days, along with the couple of days could very well be described by some as unseasonably warm and unusual weather here in the normally cold northeastern states.
In some regions of USA, 'Indian summer' is used to describe the hottest times of the year, or as the dog days, in reference to the position of Sirius, the 'Dog Star' and brightest star in the sky other than the sun.
In the southwestern United States, where frost is rare, the term is sometimes used to refer to a brief period of hot dry weather which occurs after the hottest months and before the onset of winter rains, typically in October or November.
Here in NYC, we can use it to refer to any unseasonably warm weather during the first few weeks of the winter season. Usually referring to a late blooming of something, often unexpectedly, or after it has lost relevance.