syukuzitu dakara, sinseki ga sohu no ie ni atumarimasu.

祝日 is a noun, and だから is a common way to say because it is ... after a noun.

So:

• 祝日だから = because it is a holiday

You can think of it as coming from:

• 祝日だ = it is a holiday
• 祝日だから = because it is a holiday

This is very natural Japanese.

With nouns, the plain-style pattern is:

• noun + だから

So you do not say 祝日だだから.

The だ is basically built into だから in this pattern.

Compare:

• plain: 祝日だから
• polite: 祝日ですから
• also common/polite: 祝日なので

So 祝日だから is correct, and 祝日ですから would also be correct if you want a more fully polite style.

Japanese often leaves out subjects when they are obvious from context.

In English, you need it is a holiday or today is a holiday.
In Japanese, 祝日だから is enough.

The listener naturally understands something like:

• because today is a holiday
• because it is a holiday

Japanese avoids repeating information that can be understood.

が marks the grammatical subject here:

• 親戚が集まります = the relatives gather / relatives will gather

Using が gives a neutral description of what happens.

If you used は instead:

• 親戚は祖父の家に集まります

that would make 親戚 the topic, which can sound more like:

• As for the relatives, they gather at grandfather's house
• or it may suggest contrast, depending on context

So が is a very natural choice when simply stating the event.

の connects nouns. Very often it shows possession or relationship.

So:

• 祖父の家 = grandfather's house

Literally, it is something like:

• grandfather's house
• the house of grandfather

This is one of the most basic and common uses of の.

With 集まる, に is very common because it marks the place people come together to.

• 家に集まる = gather at / gather to the house

It focuses on the destination or gathering point.

で can sometimes be used with gatherings in other contexts, but に is the standard, very natural choice with 集まる when talking about where people assemble.

So in this sentence, 祖父の家に集まります sounds very natural.

集まります is the polite non-past form of the verb 集まる.

Breakdown:

• dictionary form: 集まる
• polite form: 集まります

集まる is an intransitive verb, meaning the people themselves gather.
It is not someone gathers them.

The non-past form in Japanese can mean:

• a present/habitual action
• a future action

So here it can mean something like:

• relatives gather
• relatives will gather

depending on context.

Japanese often leaves out words like my, your, or our when the relationship is clear from context.

So:

• 祖父の家 can mean my grandfather's house
• but Japanese does not need to say my explicitly if the listener already understands whose grandfather is meant

In English, you usually need that possessive.
In Japanese, it is often omitted as unnecessary.

Both can refer to a grandfather, but the nuance is different.

• 祖父 is a more neutral, dictionary-like, somewhat formal word
• おじいさん is more everyday and conversational

Very roughly:

• 祖父 = grandfather
• おじいさん = grandpa / grandfather, depending on context

Also, おじいさん can sometimes mean an old man, so context matters.

In a sentence like this, 祖父 sounds neat and standard.

Yes, this kind of mixing is common and natural in real Japanese, especially in speech and informal writing.

Here:

• だから is plain-style
• 集まります is polite

A fully polite version might be:

• 祝日ですから、親戚が祖父の家に集まります。
• 祝日なので、親戚が祖父の家に集まります。

But the original sentence is still perfectly understandable and natural.

Here are the readings:

• 祝日 → しゅくじつ
• だから → だから
• 親戚 → しんせき
• 祖父 → そふ
• 家 → いえ
• に → に
• 集まります → あつまります

So the whole sentence is read:

しゅくじつ だから、しんせき が そふ の いえ に あつまります。