Bunny - I am always encouraging people to ask questions. Even silly ones. Heck I even indulge some of those misguided individuals what go sockless.
Warning: Spoiler! (Click to show)
This question is one of yours I have problems with - theres others the same.
Its a short short question that is almost impossible to answer - theres no context, no background, no idea of why you are asking and it seems to assume theres one right answer.
You've already seen with a question above about "flannel" in summer that the answer, like most questions in life, varies - depending.
If you want help you need to expand more.
What summer where, is it for a suit, or pants or jacket, what do you have already, do you have an opinion, are you getting a bespoke suit made, will it be lined, do you know what fresco is, ...... and so on...
It depends - if you are walking outside on a 40C day then it really doesn't matter much what you are wearing - you'll be hot.
Maybe only 42.5C inside an open weave (fresco) unlined jacket as opposed to 44C inside a lined jacket or 41.5C inside a shirt with no jacket.
Some of these things are like debating the number of angels that can fit on a pin head - fascinating - but the answer doesn't make much real difference in everyday life.
Sorry again; it will be in Melbourne's heat (my thermostat says 30 degrees as of now T_T) and for suit, will be worn over an oxford or a linen blend. Trying to find a suit where the jacket is casual enough to wear on its own and if so might be worn over a polo. Not sure if it will be lined; though it will be either MTM or RTW (still looking)