Quello che amo di settembre è l’aria mite, ma a ottobre il giardino cambia colore molto in fretta.

In this sentence, quello che means what or more literally the thing that.

So:

Quello che amo di settembre
= What I love about September
= literally, The thing that I love about September

This is a very common Italian structure:

• quello che penso = what I think
• quello che voglio = what I want
• quello che vedi = what you see

Here, the whole clause Quello che amo di settembre is the subject of the sentence.

Because di here means about or of, not in.

Compare:

• quello che amo di settembre = what I love about September
• a settembre = in September

So:

• di settembre answers about what?
• a settembre answers when?

A similar pattern is:

• La cosa che amo di questa città = The thing I love about this city

For an English speaker, this is worth remembering because English often uses about, but Italian often uses di in this kind of structure.

Yes. All three are possible, but they sound slightly different in style:

• quello che amo di settembre = the most common and neutral
• ciò che amo di settembre = a bit more formal or written
• quel che amo di settembre = shorter, a little more literary or elegant

For everyday spoken Italian, quello che is usually the safest choice.

Because the subject is treated as one thing: quello che amo di settembre.

So the structure is:

• Quello che amo di settembre = singular idea / one thing
• è l’aria mite = is the mild air

Even though l’aria mite has two words, the noun aria is singular, so è is correct.

A useful comparison:

• Quello che mi piace è la tranquillità.
• Quello che mi piacciono sono i colori.

In the second example, i colori is plural, so you may hear sono. But in your sentence, l’aria mite is singular, so è is the natural choice.

Mite means mild, gentle, or soft.

With aria, it means the air is pleasantly mild, not too cold and not too hot.

So l’aria mite means:

• mild air
• gentle air
• pleasantly temperate air

In weather descriptions, mite is a very useful adjective:

• un clima mite = a mild climate
• temperature miti = mild temperatures

Both are possible, but l’aria mite is the more normal, neutral word order.

In Italian, adjectives often come after the noun, especially when they are descriptive and specific:

• aria mite
• giornata fredda
• colore intenso

Putting the adjective before the noun can sound more literary, poetic, or emphatic:

• la mite aria

So l’aria mite is the everyday, natural choice.

Because when Italian uses months as simple time expressions, it usually does not use an article.

So:

• a ottobre = in October
• a settembre = in September
• in ottobre = also possible, though a ottobre is very common

This is different from English, where you just say in October and don’t have to think about articles. In Italian, the normal pattern is simply:

• a + month
• in + month

without il or l’.

Because in Italian, the names of months are normally written with lowercase letters, not capitals.

So:

• settembre
• ottobre
• gennaio
• luglio

They are only capitalized if they start a sentence or appear in a title. This is different from English, where months are always capitalized.

Because cambiare colore is a common Italian expression meaning to change color or to turn color.

So:

• il giardino cambia colore = the garden changes color
• le foglie cambiano colore = the leaves change color

This is a very natural way to describe seasonal changes.

Notice that colore appears without an article here. The phrase works almost like a fixed expression:

• cambiare colore
• cambiare forma
• cambiare aspetto

If you said cambia il colore, it would more likely suggest changing a specific color, as if someone were altering it deliberately.

In fretta is a fixed expression meaning quickly or in a hurry.

So:

• molto in fretta = very quickly

Here, molto is an adverb modifying the whole phrase in fretta.

By contrast, molta fretta means a lot of hurry, and it normally appears with avere:

• Ho molta fretta = I’m in a big hurry

So:

• il giardino cambia colore molto in fretta = the garden changes color very quickly
• ho molta fretta = I’m in a hurry

That is why molto in fretta is correct here, not molta fretta.

Because Italian usually drops subject pronouns when the verb ending already makes the subject clear.

So:

• amo already means I love
• cambia already tells you it is he/she/it changes

Italian is a pro-drop language, so pronouns like io, tu, lui/lei are often omitted unless they are needed for emphasis or contrast.

For example:

• Amo settembre. = I love September.
• Io amo settembre, ma lui preferisce ottobre. = I love September, but he prefers October.

In your sentence, no pronouns are needed.

Yes, a little. This structure puts emphasis on what I love about September and then identifies it.

It is similar to English sentences like:

• What I love about September is the mild air.

A more straightforward version could be:

• Amo l’aria mite di settembre.

But that does not have exactly the same focus. The original sentence is more expressive because it highlights the idea first, then gives the answer:

• What I love about September? The mild air.

So the structure is not unusual, but it is definitely chosen for emphasis and style.