O godišnjem odmoru odlučujemo zajedno, da nitko ne bude razočaran.

Godišnji odmor is a very common Croatian expression meaning annual leave, vacation time, or holiday leave.

Literally:

• godišnji = annual / yearly
• odmor = rest / break

In practice, Croatians often use it for the vacation period people take from work, especially summer holidays.

Because the preposition o usually means about and requires the locative case.

So:

• godišnji odmor = annual leave
• o godišnjem odmoru = about the annual leave

Here the sentence is talking about deciding on the vacation, so o + locative is the natural choice.

Because both words change form in the locative singular after o.

Base form:

• godišnji odmor

After o:

• o godišnjem odmoru

The endings show the case:

• godišnji → godišnjem
• odmor → odmoru

So this is simply normal case agreement after the preposition o.

Yes, but it means something different.

• o godišnjem odmoru = about the annual leave
• na godišnjem odmoru = on annual leave / being on vacation

So in this sentence, o is correct because the people are deciding about the vacation, not already on it.

Odlučujemo is 1st person plural present tense: we decide or we are deciding.

It matches the subject implied by the verb:

• (mi) odlučujemo = we decide

In this sentence, it gives the idea that the decision is made jointly as a process.

Odlučujemo sounds more like an ongoing, general, or habitual process: we decide together.

By contrast, odlučimo is more tied to a completed, single decision: we decide / we make the decision.

So:

• odlučujemo zajedno = we decide together
• odlučimo zajedno = we make the decision together

The version in your sentence feels more natural for discussing how the decision-making is done.

Zajedno is an adverb meaning together.

It tells us how the deciding is done:

• odlučujemo zajedno = we decide together

Yes, it can move:

• O godišnjem odmoru odlučujemo zajedno
• Zajedno odlučujemo o godišnjem odmoru

Both are correct. The difference is mainly emphasis and style, not basic meaning.

Here da is closer to so that than simple that.

It introduces the purpose or intended result:

• da nitko ne bude razočaran = so that nobody is disappointed

So the structure is:

• We decide together, so that nobody will be disappointed.

Because Croatian uses negative concord.

That means a negative word like nitko (nobody) normally goes together with a negative verb:

• nitko ne zna = nobody knows
• nitko ne dolazi = nobody is coming
• nitko ne bude razočaran = so that nobody is disappointed

In standard English, two negatives often cancel each other out, but in Croatian this combination is normal and required.

Because this clause does not describe a present fact. It describes a desired or expected result.

After da, Croatian often uses bude in this kind of meaning:

• da bude dobro = so that it will be good / so that things turn out well
• da nitko ne bude razočaran = so that nobody ends up disappointed

If you used je, it would sound more like a statement about what is already true, which is not the point here.

Because nitko is grammatically singular, even though its meaning is about no person at all.

So Croatian treats it like a singular subject:

• nitko ne bude
• not nitko ne budu

This is similar to English nobody is, not nobody are.

Because razočaran agrees with nitko, which is treated as masculine singular in grammar.

So:

• nitko ne bude razočaran

Even if the real group includes women or mixed genders, pronouns like nitko normally take the default masculine singular form in this kind of structure.

Yes. In standard Croatian punctuation, a subordinate clause introduced by da is separated by a comma.

So:

• O godišnjem odmoru odlučujemo zajedno, da nitko ne bude razočaran.

The comma marks the second clause and makes the sentence easier to read.

No. A very common alternative would be:

• O godišnjem odmoru odlučujemo zajedno kako nitko ne bi bio razočaran.

That also means We decide about the vacation together so that nobody will be disappointed.

Your original sentence is understandable and correct in meaning, but learners may also hear kako ... ne bi bio ... for this kind of purpose clause.