Jak na październik jest dziś gorąco.

No, it is a statement.

Here jak does not mean how. In this sentence it is part of the expression jak na + noun, which means something like:

• for
• considering
• for something/someone like this
• given that it is ...

So Jak na październik means for October or considering it is October.

Because the sentence is not just saying when it is hot. It is making a comparison with what you would normally expect in October.

• w październiku = in October
  This only gives the time.

• jak na październik = for October / considering it is October
  This adds the idea of surprise or comparison.

So the sentence is really about the weather being unusually hot by October standards.

Because jak na takes the accusative case.

The noun październik is masculine inanimate, and in the singular its accusative form is the same as its nominative form, so you still see październik.

Compare:

• jak na zimę = for winter
• jak na lato = for summer
• jak na październik = for October

So even though the form looks basic, grammatically it is accusative.

Because in Polish, when you describe the weather or a general condition, you usually use forms like gorąco, zimno, ciepło, chłodno.

So:

• Jest gorąco = It is hot
• Jest zimno = It is cold

These are not adjective forms agreeing with a noun. They function more like impersonal weather expressions.

You would use gorący when describing a noun, for example:

• gorący dzień = a hot day
• Ten dzień jest gorący = This day is hot

But for normal weather statements, gorąco is the natural choice.

In standard Polish, jest is the normal verb in this kind of sentence:

• Jest dziś gorąco
• Jest zimno
• Jest ciepło

You can sometimes hear shortened, elliptical speech like:

• Dziś gorąco
• Na dworze zimno

But those are more conversational, headline-like, or stylistically shortened. The full standard sentence uses jest.

Polish word order is flexible, and different orders change the emphasis.

Starting with Jak na październik puts the comparison first. It highlights the idea:

• for October, this is surprisingly hot

Then dziś narrows it to today.

A more neutral order could be:

• Dziś jest gorąco jak na październik.

Both are understandable, but the given sentence emphasizes the October comparison more strongly from the start.

Normally, no.

In Jak na październik jest dziś gorąco, the phrase jak na październik is integrated into the sentence, so no comma is required.

You may sometimes see commas used for extra pause or style in some texts, but in a normal sentence like this, no comma is the standard choice.

dziś means today. It simply tells you that this observation is about today.

Yes, you can usually replace it with dzisiaj:

• Jak na październik jest dziś gorąco.
• Jak na październik jest dzisiaj gorąco.

Both are correct. dziś is just shorter. In everyday Polish, both are very common.

Yes. This is a very useful pattern.

jak na + accusative is used when something is surprising, impressive, weak, strong, warm, cold, etc. for that category / in those circumstances.

Examples:

• Jak na luty jest bardzo ciepło.
  For February, it is very warm.

• Jak na dziecko mówi bardzo dojrzale.
  For a child, he/she speaks very maturely.

• Jak na swój wiek świetnie wygląda.
  For his/her age, he/she looks great.

So this sentence is a good model for a broader pattern, not just a one-off expression.

Yes, it is natural and idiomatic.

The structure jak na + time/season/month + weather expression is very normal in Polish. A native speaker would understand it immediately as:

• this is hotter than you would normally expect in October

A slightly more neutral everyday version might be:

• Dziś jest gorąco jak na październik.

But the original sentence is perfectly good Polish.