bunkasai no hi ha koutei ni oozei no hito ga atumarimasu.

A natural reading is:

ぶんかさい の ひ は こうてい に おおぜい の ひと が あつまります。

In kanji + kana: 文化祭の日は校庭に大勢の人が集まります。

Some key readings:

• 文化祭 = ぶんかさい
• 日 = ひ
• 校庭 = こうてい
• 大勢 = おおぜい
• 人 = ひと
• 集まります = あつまります

文化祭の日 is one noun phrase.

• 文化祭 = cultural festival
• 日 = day
• の links them

So 文化祭の日 literally means the day of the cultural festival.

Japanese very often uses A の B to mean B of A or B related to A.

The は marks the whole phrase 文化祭の日 as the topic.

So the structure is basically:

• 文化祭の日は = As for the day of the cultural festival...
• 校庭に大勢の人が集まります = a lot of people gather in the schoolyard

It is not just 日 by itself that is the topic; the entire phrase 文化祭の日 is.

Yes, and the nuance changes a little.

• 文化祭の日に simply marks the time: on the day of the cultural festival
• 文化祭の日は makes that day the topic: as for the day of the cultural festival

So は can feel slightly more like setting the scene, and sometimes it can suggest contrast, such as on that day, unlike other days...

This is a very common question.

With 集まる, the place where people gather is usually marked with に. It shows the place people come to or end up at.

• 校庭に集まる = gather in/at the schoolyard

By contrast, で often marks the place where an action is carried out.

• 校庭で遊ぶ = play in the schoolyard
• 校庭でイベントをする = hold an event in the schoolyard

So for gather, に is the natural choice.

大勢 means a large number, a crowd, or many people.

In 大勢の人, 大勢 is functioning like a noun modifying another noun, so it uses の:

• 大勢の人 = a large number of people

This is similar to how Japanese often links nouns with の.

Yes. Both are natural, but the nuance is a little different.

• 大勢の人が集まります = a large number of people gather
• 人が大勢集まります = people gather in large numbers

The first treats a large number of people as one noun phrase. The second treats 大勢 more like an adverbial expression describing how people gather.

Because 人 is the subject of 集まります.

集まる is an intransitive verb meaning to gather. The people who gather are marked with が:

• 人が集まる = people gather

So in this sentence:

• 大勢の人が = a lot of people as the subject
• 集まります = gather

集まります is polite non-past.

In Japanese, the non-past can mean:

• present/habitual: people gather
• future: people will gather

In this sentence, it most naturally sounds like a general or habitual statement about what happens on cultural festival day. Depending on context, it could also describe an upcoming event.

Because 集まる and 集める are different verbs.

• 集まる = something gathers / comes together
• 集める = someone gathers / collects something

Here, the people themselves are gathering, so 集まる is correct.

• 人が集まる = people gather
• 先生が人を集める = the teacher gathers the people

校庭 means the schoolyard or school grounds, especially the outdoor open area of a school.

It is more specific than just yard. In many school contexts, it suggests the open area where students assemble, play sports, or hold outdoor events.

Japanese usually does not use articles like the or a, and it often does not mark plural explicitly.

So:

• 人 can mean person or people, depending on context
• 大勢の already makes it clear that more than one person is meant

This is very normal in Japanese. Context does a lot of the work.

The basic meaning can stay the same even if parts move around, because the particles show each word’s role.

For example, this is also possible:

校庭に大勢の人が文化祭の日は集まります。

But it sounds less natural in most situations. The original sentence is smoother because it starts with the topic:

• 文化祭の日は = topic/time setting
• 校庭に = place
• 大勢の人が = subject
• 集まります = verb

Japanese word order is flexible, but some orders sound more natural than others.