Indian summer is an idiom (Cast. “frase hecha, refrán”; Cat. “dita”). It means that it is hot or warm and sunny when it’s supposed to be cold and cloudy or rainy. It happens in autumn. In Spanish we say “Veranillo de San Martín” and in Catalan “Estiuet de Sant Martí”, because it’s typically warm and sunny during the days near St. Martin’s Day or Martinmas, (November 11th). For instance:
JOHN: It’s being really hot this month!
PHIL: I see what you mean… Too hot to be November… But remember that in a few days it will be St. Martin 😉
JOHN: Of course!!! We’re having a beautiful Indian summer!
You can get more info about Indian summer at Wikipedia.