Probably never! Apart from a very few fixed French expressions that include être in passé simple (eg Ce fut un honneur: it was an honor) that also can be, and are, said in passé composé, passé simple is almost exclusively a literary form (including children's books) and not used in routine speech or otherwise 'normal' writing. If you are at A2 level, unless you are trying to read French literature outside of this course, you can safely ignore passé simple for now. If you have need of some knowledge of it now, read the link below, and then it's just a matter of learning the conjugations to recognise it -
https://french.kwiziq.com/revision/grammar/recognise-etre-in-le-passe-simple
