Pada Juli, keluarga besar kami akan berkumpul di rumah nenek.

Here, pada Juli means in July.

In Indonesian, pada is often used with time expressions, especially in more standard or formal language. So pada Juli is a natural way to say in July.

Di is mainly for places, like di rumah = at home / in the house.
You may hear di bulan Juli in casual speech, but pada Juli is more standard in careful writing.

Because month names are capitalized in Indonesian, just like in English.

So:

• Juli = July
• Agustus = August
• Desember = December

Keluarga besar literally looks like big family, but it usually means extended family.

So it refers not just to parents and children, but also relatives such as:

• grandparents
• aunts
• uncles
• cousins
• and other family members

This is a very common fixed expression in Indonesian.

In Indonesian, adjectives usually come after the noun.

So:

• rumah besar = big house
• keluarga besar = literally big family

That said, in this sentence keluarga besar is better understood as a set phrase meaning extended family, not just a family that happens to be large.

This is an important Indonesian distinction:

• kami = we, but not including the listener
• kita = we, including the listener

So keluarga besar kami means our extended family, but the person being spoken to is not included in that we.

If the listener were part of that family group, kita might be used in a different sentence.

Akan marks the future, similar to will in English.

So:

• akan berkumpul = will gather

Indonesian does not always have to mark future time, especially if the time is already clear from context. Since Pada Juli already shows a future time, the sentence could sometimes work without akan. But adding akan makes the future meaning extra clear and natural.

Berkumpul is the standard verb meaning to gather or to assemble.

The prefix ber- often forms intransitive verbs, and here it gives the sense of people gathering together.

• berkumpul = standard / neutral
• kumpul = common in casual speech, but less formal

So in a full, standard sentence, berkumpul is the better choice.

Di rumah nenek literally means at grandmother’s house.

Breakdown:

• di = at / in
• rumah = house
• nenek = grandmother

Indonesian often shows possession by simply putting two nouns together:

• rumah nenek = grandmother’s house
• buku saya = my book
• mobil ayah = father’s car

So there is no apostrophe like in English. The relationship is understood from the word order.

It could say rumah nenek kami, and that would mean our grandmother’s house more explicitly.

But di rumah nenek is already very natural if the context makes it obvious whose grandmother is meant. Since the sentence already has keluarga besar kami, many speakers would feel that nenek is understood.

So:

• di rumah nenek = natural, less explicit
• di rumah nenek kami = more explicit

Both can work.

Indonesian is fairly flexible about word order, especially with time expressions.

Putting Pada Juli at the beginning highlights the time frame first. That is very natural.

You could also say: Keluarga besar kami akan berkumpul di rumah nenek pada Juli.

Both are grammatical. The version with Pada Juli first just gives a slightly stronger focus to when the event will happen.

In this sentence, di is a preposition meaning at or in.

So:

• di rumah nenek = at grandmother’s house

This is different from di- as a verb prefix in passive verbs. For example:

• dibuka = is opened

A quick way to tell the difference:

• if di is written separately, it is usually a preposition
• if di- is attached to a verb, it is usually a passive prefix

Here it is separate: di rumah, so it is definitely the preposition.

Usually pada Juli means in July, meaning sometime during July. It does not necessarily mean the whole month.

So this sentence most naturally suggests that the family gathering will happen at some point in July.

If you wanted to be more exact, you could say something like:

• pada tanggal 12 Juli = on July 12
• selama bulan Juli = throughout July / during the month of July