Vikendom si ponekad dopustimo skupi ručak u restoranu kad ne moramo sami ništa pržiti.

All three are possible, but they differ slightly in style and nuance.

• Vikendom is the most natural here.

  • It’s the instrumental singular used adverbially: literally “by weekend / on weekends”.
  • Very common for talking about regular, repeated actions: Vikendom dugo spavamo. – We sleep in on weekends.

• Za vikend is also correct, but it often feels more like “on the weekend / this weekend” (more like a single, upcoming or specific weekend), though it can be generic too:

  • Za vikend idemo na more. – We’re going to the seaside this weekend.

• Vikendima is the plural instrumental: literally “on (the) weekends”.

  • Also okay, but a bit less common in everyday speech than vikendom:
  • Vikendima radim. – I work on weekends.

In your sentence, Vikendom is the natural and typical choice for a habitual action.

Si here is a reflexive pronoun in the dative case and means “to ourselves”.

• dopustiti (nekome) nešto = to allow (someone) something / to permit (someone) something / to treat oneself to something
  
  • Dopustimo si skupi ručak. – We allow ourselves an expensive lunch.

Se is the accusative or dative reflexive clitic, but in practice:

• si is used as the dative reflexive clitic with 1st and 2nd person:
  • Ja si dopustim… – I allow myself…
  • Mi si dopustimo… – We allow ourselves…
• se is much more general, especially for 3rd person:
  • On si / sebi dopusti… and On se dopusti… can both appear, but patterns vary by verb and speaker.

In this specific phrase, dopustiti si (nešto) is a set pattern meaning to treat oneself to something, and si is required to express “to oneself”.
Without si, dopustimo skupi ručak would sound like we allow an expensive lunch (to someone/something unspecified), which is incomplete or odd here.

Croatian has relatively flexible word order, but clitics (short unstressed words like si, se, je, sam, mi) follow special rules.

• Si is a clitic and must appear in the so‑called second position in the clause:
  • It usually comes right after the first stressed word or phrase.

In the sentence:

• First stressed element: Vikendom
• Next position is the clitic slot → si
• Then other adverbs and the verb: ponekad dopustimo

So:

• ✅ Vikendom si ponekad dopustimo… – correct and natural.
• ❌ Vikendom ponekad si dopustimo… – ungrammatical, because si is no longer in second position.

You can, however, move ponekad:

• Ponekad si vikendom dopustimo skupi ručak… – also correct
  
  • Here Ponekad is first, so si comes right after it.

The key is: clitics like si must come very early, right after the first stressed element.

Grammatically, we have:

• ručak – masculine singular, accusative (object of dopustimo)
• The adjective must agree: masculine singular, accusative, inanimate.

For an inanimate masculine noun, nominative and accusative look the same:

• short form: skup ručak (nom/acc)
• long/definite form: skupi ručak (nom/acc)

Both skup ručak and skupi ručak are grammatically possible. The difference is stylistic:

• skup ručak – short form; a bit more neutral or “bare”.
• skupi ručak – long/definite form; often used in everyday speech, can sound slightly more specific or emphatic, like “an expensive kind of lunch”.

Skupog ručka would be genitive (of an expensive lunch), which is not needed here because ručak is a direct object, not an object of a preposition or a quantity.

So skupi ručak is correct and natural; skup ručak would also be understood and acceptable in many contexts.

Ponekad means “sometimes” and modifies the whole action dopustimo si skupi ručak….

In your sentence, Vikendom si ponekad dopustimo… means:

• On weekends, we sometimes allow ourselves an expensive lunch…

You can move ponekad quite freely:

• Ponekad si vikendom dopustimo skupi ručak…
• Vikendom ponekad si dopustimo skupi ručak… – ungrammatical, because si must be second
• Vikendom si dopustimo ponekad skupi ručak… – possible, but less natural; it makes ponekad sound like it’s focusing on the lunch itself in a slightly odd way.

The most natural positions are:

• Ponekad si vikendom dopustimo…
• Vikendom si ponekad dopustimo…

In all these, ponekad simply expresses that this habit happens occasionally, not every weekend.

Kad and kada mean the same: “when”.

• kada – full form, slightly more formal or careful
• kad – shortened, very common in everyday spoken and written Croatian

In your sentence:

• …kad ne moramo sami ništa pržiti.
• …kada ne moramo sami ništa pržiti.

Both are correct; the meaning is identical. The short kad simply sounds more colloquial and is very typical in speech.

Ne moramo sami ništa pržiti breaks down as:

• ne moramo – we don’t have to / we need not
• sami – by ourselves, on our own
• ništa – literally nothing, but with a negated verb it corresponds to English anything
• pržiti – to fry

So the phrase means:

• when we don’t have to fry anything ourselves
  (literally: when we don’t have to, ourselves, nothing fry)

In Croatian, double negation is normal and required:

• Ne moramo ništa pržiti. – We don’t have to fry anything.
• Ne vidim ništa. – I don’t see anything.
• Nemam ništa. – I have nothing / I don’t have anything.

Using a positive word (nešto “something”) with a negative verb is usually wrong in standard Croatian in this kind of sentence.

Sami adds emphasis: they don’t have to do the frying themselves, which is part of the pleasure of going to a restaurant.

The verb morati (must, have to) is followed by the infinitive:

• morati + infinitive → moramo pržiti – we have to fry

Because the sentence is negated:

• ne moramo pržiti – we don’t have to fry

Pržiti specifically means “to fry” (in oil/fat). So the line:

• …kad ne moramo sami ništa pržiti.
  emphasizes that they don’t have to do any frying themselves.

You could replace pržiti with kuhati (to cook/boil):

• …kad ne moramo sami ništa kuhati. – when we don’t have to cook anything ourselves.

This would slightly change the meaning: instead of focusing on frying, it refers to cooking in general. Grammatically both are fine; the verb choice just changes the nuance.

U restoranu means “in a restaurant” and is in the locative case after the preposition u.

• Vikendom si ponekad dopustimo skupi ručak u restoranu…
  – On weekends we sometimes allow ourselves an expensive lunch in a restaurant…

If you omit u restoranu, the sentence becomes:

• Vikendom si ponekad dopustimo skupi ručak, kad ne moramo sami ništa pržiti.
  – On weekends we sometimes allow ourselves an expensive lunch when we don’t have to fry anything ourselves.

This is still grammatical, but it no longer explicitly says where – it could be at home, in a hotel, etc. U restoranu clarifies that the treat is specifically about eating out.

Word order options (all natural):

• …dopustimo si skupi ručak u restoranu…
• …dopustimo si u restoranu skupi ručak… (slightly marked; focus shifts a bit to “in a restaurant”)
• U restoranu si vikendom ponekad dopustimo skupi ručak… (strong emphasis on at the restaurant).

Meaning stays similar, but shifting u restoranu forward can give it more emphasis.

Dopustiti is a perfective verb (“to allow / to permit / to treat oneself” as a completed act). Normally:

• Perfective present often refers to future or single, complete events:
  • Dopustit ću si skupi ručak. – I’ll allow myself an expensive lunch.

However, in Croatian, perfective verbs in the present can be used with adverbs of frequency or time (vikendom, ponekad, svake subote) to express repeated, habitual actions, like “each time we complete this action”:

• Vikendom si ponekad dopustimo skupi ručak…
  → On weekends we sometimes (on those occasions) allow ourselves an expensive lunch…

You could also use the imperfective dopuštati:

• Vikendom si ponekad dopuštamo skupi ručak…

This is also correct and puts a bit more focus on the ongoing, repeated nature. Many speakers freely use the perfective in such “iterative” contexts, especially with clear time markers like vikendom.

Croatian is a pro-drop language: subject pronouns (ja, ti, on, mi, vi, oni) are often omitted when the verb ending already shows the person and number.

• dopustimo is 1st person plural (we):
  • ja dopustim – I allow
  • ti dopustiš – you allow
  • on/ona dopusti – he/she allows
  • mi dopustimo – we allow
  • vi dopustite – you (pl/formal) allow
  • oni dopuste – they allow

So dopustimo already tells you the subject is “we”. Adding mi:

• Mi si vikendom ponekad dopustimo skupi ručak…

is grammatically correct, but mi would sound emphatic, like stressing “we (as opposed to someone else) do this”.

In neutral sentences, speakers usually leave mi out when it’s clear from the verb.

Yes, several idiomatic alternatives express a similar idea of “treating oneself”:

• priuštiti si skupi ručak
  – to afford oneself / treat oneself to an expensive lunch

  • Vikendom si ponekad priuštimo skupi ručak u restoranu.

• počastiti se skupim ručkom
  – literally to honour oneself with an expensive lunch → to treat oneself to an expensive lunch

  • Vikendom se ponekad počastimo skupim ručkom u restoranu.

• otići na skupi ručak
  – to go for an expensive lunch (less explicit about “treating ourselves”, but often implies it)

  • Vikendom ponekad odemo na skupi ručak u restoran.

All of these are natural; your original dopustiti si skupi ručak focuses on the idea of “allowing yourself a luxury” from time to time.