На праздник мы купим свечи и один большой шарик для младшего брата.

Here на праздник means for the holiday / for the celebration / for the occasion.

Russian often uses на + accusative with events to show the intended occasion:

• купить торт на день рождения = buy a cake for a birthday
• надеть новое платье на праздник = wear a new dress for a celebration

If you said в праздник, that would usually mean on the holiday / during the holiday, focusing more on time.

Because the preposition на often takes the accusative when it points to a goal, target, or intended occasion.

Here the idea is not physical motion onto something, but a kind of figurative target: for the celebration.

Also, праздник is a masculine inanimate noun, and in the singular its accusative form is the same as its nominative form. So you see праздник, not a visibly different ending.

Купим is the 1st person plural future of the perfective verb купить.

Russian uses the perfective future when the speaker sees the action as a completed result:

• мы купим = we will buy

That fits this sentence well, because the idea is a single planned purchase with a clear outcome.

If you used будем покупать, that would sound more like:

• a process,
• repeated buying,
• or buying without focusing on completion.

Here свечи is accusative plural, because it is the direct object of купим.

But for inanimate plural nouns, the accusative is usually identical to the nominative. So:

• nominative plural: свечи
• accusative plural: свечи

That is why the form looks unchanged.

Yes, in this context it very likely means balloon.

Literally, шарик is a diminutive form of шар, which means ball or sphere, so шарик can mean:

• a little ball
• a balloon

But with свечи and на праздник, a native speaker will normally understand шарик as a party balloon.

Mostly it is the literal number one.

Russian has no articles like a/an/the, so sometimes один can feel a little article-like in some contexts. But in this sentence it is best understood as the numeral:

• свечи = candles
• один большой шарик = one big balloon

So the sentence is clearly counting the items.

All three words agree with шарик.

Since шарик is:

• masculine
• singular
• inanimate
• accusative here

the other words match it:

• один
• большой
• шарик

For masculine inanimate nouns in the singular, the accusative looks like the nominative, so the forms stay:

• один
• большой
• шарик

If the noun were feminine, you would get something like:

• одна большая свеча

Because the preposition для requires the genitive case.

So:

• брат → брата
• младший → младшего

That gives:

• для младшего брата

This whole phrase means for the younger brother and describes who the balloon is intended for.

Both are possible, but they are structured a little differently.

• купим младшему брату шарик = we will buy the younger brother a balloon
  This presents младшему брату more directly as the beneficiary.

• один большой шарик для младшего брата = one big balloon for the younger brother
  This treats для младшего брата as part of the noun phrase, describing the balloon itself.

In this sentence, that wording helps group the ideas neatly:

• candles
• and one big balloon for the younger brother

Because младший брат is the normal Russian expression for younger brother.

Russian uses:

• старший брат = older brother
• младший брат = younger brother

By contrast, молодой брат would mean a young brother, focusing on age, not specifically on being younger than another sibling.

Russian often leaves out possessive words when the relationship is obvious from context.

Since the sentence already has мы, a listener may naturally understand для младшего брата as something like for our younger brother, depending on the situation.

If the speaker wanted to be more explicit, they could say:

• для нашего младшего брата

But it is not always necessary.

Yes. Russian word order is flexible, and changing it usually changes emphasis, not the basic meaning.

For example:

• На праздник мы купим свечи... puts на праздник first, setting the scene.
• Мы купим на праздник свечи... sounds a bit more neutral.
• Для младшего брата мы купим один большой шарик would emphasize who the balloon is for.

Because Russian marks relationships with case endings, the sentence can move around more freely than English.