Rujan mi je uvijek zanimljiv, a ono što me u listopadu posebno veseli jesu mirne večeri kod kuće.

In Croatian, names of months are normally written with a lowercase letter: rujan, listopad, studeni, and so on.

Here, Rujan is capitalized only because it is the first word of the sentence. If it appeared in the middle of the sentence, it would usually be rujan.

So the contrast is not grammatical; it is just standard capitalization:

• Rujan mi je uvijek zanimljiv.
• U rujnu je još toplo.

mi is the unstressed dative form of ja, so it means to me.
je is is.

So literally, Rujan mi je uvijek zanimljiv is something like:

• September is always interesting to me

A very natural English equivalent would be I always find September interesting.

This is a very common Croatian pattern:

• Knjiga mi je dobra. = The book is good to me / I think the book is good
• Film mi je dosadan. = I find the film boring

So mi marks the person experiencing the opinion or impression.

Because it agrees with rujan.

In Croatian, adjectives agree with the noun they describe in:

• gender
• number
• case

rujan is masculine singular, so the adjective is also masculine singular:

• zanimljiv rujan or Rujan je zanimljiv

Compare:

• zanimljiva knjiga — feminine singular
• zanimljivo selo — neuter singular
• zanimljivi ljudi — masculine plural

So zanimljiv is simply the correct agreement form for rujan.

Because after u meaning in for time expressions like months, Croatian uses the locative case.

So:

• listopad = dictionary form, nominative
• u listopadu = locative, meaning in October

This is the normal pattern with months:

• u siječnju
• u veljači
• u ožujku
• u rujnu
• u listopadu

The preposition u can take different cases depending on meaning:

• u + locative = in, at
• u + accusative = into, to

Here it is a time expression, so u listopadu = in October.

In this sentence, a links two full clauses. It often means something like:

• and
• while
• whereas
• as for

It is not exactly the same as i and not as strong as ali.

Very roughly:

• i = simple and
• ali = but
• a = a softer contrast or a shift to another related point

So here the sentence moves from September to October:

• Rujan mi je uvijek zanimljiv, a ...
• September is always interesting to me, and/as for October ...

The comma is standard because a is joining two independent clauses.

ono što is a very common Croatian structure meaning:

• what
• that which
• the thing that

Grammatically:

• ono = that, neuter singular pronoun
• što = relative pronoun, what/that

Together they introduce a noun-like clause. In this sentence:

• ono što me u listopadu posebno veseli

means something like:

• what especially makes me happy in October
• the thing that especially pleases me in October

You will see this pattern a lot:

• Ono što želim je mir. = What I want is peace.
• Ono što znam nije dovoljno. = What I know is not enough.

Because the verb veseliti works differently from some English verbs.

With veseliti, the thing causing joy is the grammatical subject, and the person feeling joy is in the accusative:

• To me veseli. = That makes me happy.
• Mirne večeri me vesele. = Quiet evenings make me happy.

So:

• me = accusative me
• not mi = dative to me

This is important because English learners often compare it to verbs like sviđati se, which use the dative:

• To mi se sviđa. = I like that.

So:

• veseliti → me
• sviđati se → mi

In your sentence, ono što me u listopadu posebno veseli literally means what especially makes me happy in October.

posebno is an adverb meaning:

• especially
• particularly

It modifies the verb veseli. So the idea is:

• what especially pleases/makes me happy in October

Its position is quite natural here:

• ono što me u listopadu posebno veseli

Croatian word order is flexible, so you might also see:

• ono što me posebno veseli u listopadu
• ono što me u listopadu veseli posebno

But the version in your sentence sounds smooth and natural, with posebno directly before the verb it modifies.

This is a very good question.

The phrase begins with ono što, which looks singular because ono is singular neuter. But the sentence is identifying what that something is, and the identified thing is mirne večeri kod kuće, which is plural.

So Croatian often uses plural agreement in this kind of sentence:

• Ono što me veseli jesu mirne večeri.

This is very similar to English:

• What makes me happy are quiet evenings at home.

The verb agrees with the plural idea being identified. That is why jesu sounds natural here.

You may sometimes encounter singular agreement in related structures, especially in less formal usage, but jesu is a very normal and standard choice here because mirne večeri is plural.

Here mirne večeri is nominative plural.

Why? Because it is the noun phrase identified after jesu:

• jesu mirne večeri kod kuće

The noun is:

• večer = evening

Its nominative plural is:

• večeri

The adjective must agree with it:

• mirna večer = a quiet evening
• mirne večeri = quiet evenings

So:

• mirne = feminine plural nominative
• večeri = feminine plural nominative

This is not an object here; it is the phrase being equated with ono što me u listopadu posebno veseli.

kod kuće is an idiomatic expression meaning at home.

Grammatically:

• kod is a preposition that takes the genitive
• kuće is the genitive singular of kuća

Literally, kod often means something like at someone’s place or by:

• kod liječnika = at the doctor’s
• kod prijatelja = at a friend’s place

But kod kuće is a fixed everyday expression for at home.

So in this sentence, mirne večeri kod kuće means quiet evenings at home.

A learner should be careful not to interpret it too literally as near the house. It simply means being at home.

Because Croatian has much freer word order than English.

English usually depends heavily on word order to show who does what. Croatian relies much more on case endings and verb forms, so speakers can move elements around for:

• emphasis
• rhythm
• style
• topic-focus structure

For example:

• Rujan mi je uvijek zanimljiv puts Rujan first as the topic.
• ono što me u listopadu posebno veseli builds suspense before telling you what that thing is.
• jesu mirne večeri kod kuće gives the answer at the end for emphasis.

A more straightforward version of the second idea could be:

• U listopadu me posebno vesele mirne večeri kod kuće.

That is also natural, but the original sounds more literary or reflective.

Because they are clitics, and Croatian clitics usually go near the beginning of their clause, often in second position.

That is why you get:

• Rujan mi je uvijek zanimljiv not normally
• Rujan uvijek zanimljiv mi je

And:

• ono što me u listopadu posebno veseli with me in its normal clitic position inside that clause.

These short unstressed forms include things like:

• mi
• ti
• se
• ga
• je
• su

They do not behave like full stressed words. Their placement is one of the trickier parts of Croatian for English speakers, but in this sentence the placement is completely normal and natural.

Yes. A simpler, more direct version would be something like:

• Rujan mi je uvijek zanimljiv, a u listopadu me posebno vesele mirne večeri kod kuće.

This says almost the same thing, but with a more straightforward structure:

• u listopadu first
• me posebno vesele as the main verb phrase
• mirne večeri kod kuće clearly as the subject

The original version with ono što ... jesu ... is a bit more elegant and more expressive. It draws attention to the final phrase mirne večeri kod kuće by delaying it until the end.