Lokakuu on pimeä kuukausi, mutta syysloma on lähellä.

Because it is the first word of the sentence.

In Finnish, month names are not normally capitalized the way they are in English. So:

• lokakuu = October
• syysloma = autumn holiday / fall break

They only get a capital letter at the start of a sentence or in a title.

On is the present-tense form of olla, which means to be.

Here it appears twice because both clauses use is:

• Lokakuu on pimeä kuukausi
• syysloma on lähellä

Finnish uses the same form on for all third-person singular subjects, such as:

• hän on = he/she is
• se on = it is
• lokakuu on = October is
• syysloma on = the autumn break is

Finnish does not have articles like English a/an and the.

So pimeä kuukausi can mean:

• a dark month
• the dark month

The exact meaning depends on context. In this sentence, English naturally uses a dark month.

Pimeä is an adjective meaning dark, and kuukausi means month.

In Finnish, adjectives usually come before the noun, just like in English:

• pimeä kuukausi = dark month
• pitkä päivä = long day
• kylmä talvi = cold winter

Also, adjectives agree with the noun in number and case. Here both words are in the singular nominative:

• pimeä = nominative singular
• kuukausi = nominative singular

Because this sentence is identifying or describing what October is:

• Lokakuu on pimeä kuukausi = October is a dark month

After olla (to be), a singular noun like this is often in the nominative when it names what something is.

So kuukausi stays in its basic dictionary form.

This is a very common pattern:

• Hän on opettaja = He/She is a teacher
• Tämä on hyvä idea = This is a good idea
• Lokakuu on pimeä kuukausi = October is a dark month

Syysloma is a compound noun.

It is made from:

• syys- = autumn-
• loma = holiday, vacation, break

So syysloma literally means something like autumn holiday.

Finnish uses compound words very often, and they are usually written as one word, not two. Compare:

• syysloma = autumn break
• kesäloma = summer holiday
• joululoma = Christmas holiday

Because compounds often use a stem form rather than the full dictionary form.

The dictionary form is:

• syksy = autumn

But in compounds, Finnish usually uses:

• syys-

So you get:

• syysloma = autumn break
• syyskuu = September
• syyssade = autumn rain

This is something learners usually just get used to over time: the first part of a compound is not always identical to the standalone dictionary form.

Lähellä means near, close by, or in some contexts approaching soon.

Grammatically, it can behave like an adverb or a postposition.

In this sentence, it stands on its own:

• syysloma on lähellä = the autumn break is near / coming soon

So here it works very much like an adverbial expression.

Yes.

It can refer to:

• physical closeness: something is nearby
• time: something is coming soon
• sometimes even more abstract closeness

In this sentence, syysloma on lähellä is about time, not location. It means the break is not far away in the calendar.

When lähellä is used with a noun, that noun is usually in the genitive:

• koulun lähellä = near the school
• talon lähellä = near the house
• kaupungin lähellä = near the city

So:

• Syysloma on lähellä = The autumn break is near
• Koulu on kirkon lähellä = The school is near the church

In your sentence, there is no following noun, so lähellä simply means near / soon by itself.

Mutta means but.

It joins two main clauses:

• Lokakuu on pimeä kuukausi
• mutta syysloma on lähellä

Unlike in some languages, mutta does not force a special word order change here. The second clause keeps the normal order:

• subject + verb + other information

So:

• syysloma = subject
• on = verb
• lähellä = complement/adverbial

Yes. This is a very natural, neutral Finnish sentence.

The usual basic order is:

• subject + verb + complement

So:

• Lokakuu on pimeä kuukausi
• syysloma on lähellä

Finnish word order is somewhat flexible, but this version sounds straightforward and natural for ordinary statements.