Ono što mi se u siječnju najviše sviđa jest tišina poslije blagdana.

Ono što means that which / what.

So:

• Ono što mi se u siječnju najviše sviđa = What I like most in January
• jest tišina poslije blagdana = is the quiet after the holidays

This is a very common Croatian pattern:

• Ono što trebam jest odmor. = What I need is rest.
• Ono što želim jest mir. = What I want is peace.

Here, ono is a neuter pronoun meaning that, and što introduces the clause what/that which.

Croatian often uses sviđati se for to be pleasing to, which works differently from English to like.

• Sviđa mi se X literally means X is pleasing to me
• So Tišina mi se sviđa = I like the quiet

In this sentence:

• mi = to me
• se = reflexive particle used with sviđati (se)
• sviđa = is pleasing

So ono što mi se najviše sviđa is literally that which is most pleasing to me.

By contrast, voljeti means to love / really like, but it is not used in exactly the same way in all contexts.

Mi is the unstressed dative form of ja (I / me), and here it means to me.

Compare:

• Meni se sviđa ova knjiga. = I like this book.
• Mi se sviđa ova knjiga. = same meaning, but with the shorter unstressed form

So in the sentence:

• mi se sviđa = is pleasing to me

Croatian often uses the dative with verbs of liking, seeming, needing, and similar ideas.

Se is a reflexive particle that belongs to the verb sviđati se.

You usually learn this verb together with se:

• sviđati se = to be pleasing / to be liked

So you do not normally say just sviđati in this meaning. The natural pattern is:

• Sviđa mi se film. = I like the film.
• Sviđaju mi se filmovi. = I like the films.

In short, se is simply part of how this verb is used.

After u when talking about being in a time period or location, Croatian uses the locative case.

• siječanj = January
• u siječnju = in January

So siječnju is the locative singular form of siječanj.

More examples:

• u veljači = in February
• u ožujku = in March
• u gradu = in the city

Here it answers the question when? — in January.

Yes, najviše is the superlative form of mnogo / mnogo toga / puno in an adverbial sense, and here it means most.

In this sentence it modifies the idea of liking:

• najviše sviđa = likes most

So:

• Ono što mi se u siječnju najviše sviđa... = What I like most in January...

Compare:

• Volim ga najviše. = I love him most.
• Najviše mi se sviđa plava boja. = I like blue best.

The verb agrees with the thing that is pleasing, not with the person who likes it.

Here the subject is the clause:

• Ono što mi se u siječnju najviše sviđa = what I like most in January

That whole clause is treated as singular, so the verb is singular:

• sviđa

Compare:

• Sviđa mi se tišina. = singular, because tišina is singular
• Sviđaju mi se blagdani. = plural, because blagdani is plural

So sviđa is singular because the grammatical subject is singular.

Both jest and je are forms of the verb biti (to be).

In this sentence, jest is a fuller, slightly more formal or emphatic form:

• Ono što mi se u siječnju najviše sviđa jest tišina poslije blagdana.
• Ono što mi se u siječnju najviše sviđa je tišina poslije blagdana.

Both are correct.

Using jest often sounds a bit more careful, written, or stylistically polished, especially in sentences of the type What X is, is Y.

Because tišina is the predicate noun after jest / je.

The structure is:

• Ono što mi se... najviše sviđa = subject-like clause
• jest tišina poslije blagdana = what it is

So tišina stays in the nominative singular:

• tišina = quiet / silence

This is normal after forms of to be in Croatian.

After the preposition poslije (after), Croatian uses the genitive case.

• blagdani = holidays (nominative plural)
• blagdana = of the holidays / after the holidays (genitive plural)

So:

• poslije blagdana = after the holidays

Other examples:

• poslije ručka = after lunch
• poslije škole = after school
• poslije praznika = after the vacation/holidays

Blagdani means holidays, especially feast days / public or religious holidays. In this January context, it strongly suggests the Christmas and New Year holiday period.

Praznici can also mean holidays, but often in the sense of school holidays, vacation, or a break period more generally.

So:

• blagdani = festive holidays, often official or religious
• praznici = holidays/breaks/vacation time

In this sentence, blagdana fits very naturally because it refers to the holiday season that has just ended.

Yes, Croatian word order is more flexible than English, though not completely free. This sentence uses a very natural, polished order.

The given version:

• Ono što mi se u siječnju najviše sviđa jest tišina poslije blagdana.

You might also hear or see variations like:

• Ono što mi se najviše sviđa u siječnju jest tišina poslije blagdana.
• Tišina poslije blagdana jest ono što mi se u siječnju najviše sviđa.

These all mean roughly the same thing, but the emphasis changes.

The original order is good because it first introduces the topic what I like most in January, then gives the answer the quiet after the holidays.

Both are possible, depending on context.

• tišina can mean silence
• but in many everyday sentences it is more naturally translated as quiet

Here the quiet after the holidays sounds more natural in English than the silence after the holidays, unless you want a more poetic tone.

So the Croatian word itself can cover both ideas.

Yes. Both are correct:

• poslije blagdana
• nakon blagdana

Both mean after the holidays and both take the genitive.

The difference is mostly stylistic:

• nakon can sound slightly more formal or neutral
• poslije is very common in everyday language

So the sentence could also be:

• Ono što mi se u siječnju najviše sviđa jest tišina nakon blagdana.

That would still be perfectly natural.

A more casual way might be:

• U siječnju mi se najviše sviđa tišina poslije blagdana.

This is simpler and more direct:

• In January, what I like most is the quiet after the holidays

The original sentence with Ono što ... jest ... sounds a bit more deliberate, elegant, or written. Both are correct; the difference is mainly style.