hanami no hi ha, kouen ni oozei no hito ga kimasu.

花見の日 is a noun phrase made with noun + の + noun.

• 花見 = flower viewing, usually cherry-blossom viewing
• 日 = day

So 花見の日 means the day of flower viewing or the day for cherry-blossom viewing.

The particle の connects nouns and often works like of, 's, or for in English. It shows that 日 is being specified by 花見.

Examples:

• 試験の日 = exam day / the day of the exam
• 休みの日 = day off

Because 花見の日 is the full topic of the sentence, and は marks that topic.

So the structure is:

• 花見の日 = on flower-viewing days
• は = as for / speaking of

This means the sentence is talking about what happens on flower-viewing days.

It does not mean only 日 by itself is the topic. The whole phrase 花見の日 is the topic.

Here, に marks the destination of movement.

Since 来ます means come, the place marked by に is the place people come to.

So:

• 公園に来ます = come to the park

This is similar to:

• 学校に行きます = go to school
• うちに来ます = come home / come to my house

大勢の人 means a large number of people or many people.

• 大勢 = a crowd / a large number of people
• 人 = people

The の links 大勢 to 人, so 大勢の人 literally means something like a large-number group of people.

In natural English, you would usually say:

• many people
• a lot of people
• a large crowd

Japanese often uses の to connect this kind of modifying expression to a noun.

が marks 人 as the subject of 来ます.

So:

• 大勢の人が来ます = a lot of people come

Using が here is natural because it presents a lot of people as the thing that does the action.

If you used は instead, it would sound more contrastive or would shift the topic:

• 大勢の人は来ます could suggest As for a lot of people, they do come... perhaps contrasting with someone else who does not.

In this sentence, 花見の日は is already the topic, so が works well for the subject inside that topic frame.

Japanese 来ます is a non-past form. It can express:

• present/habitual action
• future action
• general facts

In this sentence, it most naturally expresses a habitual or general occurrence:

• On flower-viewing days, lots of people come to the park.

It is not necessarily talking about just one specific future day. It sounds more like a general pattern.

Not grammatically necessary, but it is natural in writing.

The comma shows a pause after the topic:

• 花見の日は、公園に大勢の人が来ます。

It helps the reader separate:

• the topic: 花見の日は
• the rest of the statement: 公園に大勢の人が来ます

You may also see the sentence written without the comma.

Not completely. Japanese word order is fairly flexible because particles show each word’s role.

This sentence can be rearranged without changing the basic meaning very much, for example:

• 花見の日は、公園に大勢の人が来ます。
• 花見の日は、大勢の人が公園に来ます。

Both are natural.

The version with 公園に earlier may slightly emphasize the location first. The version with 大勢の人が earlier may bring the people into focus first.

The verb 来ます usually stays near the end.

Yes. In everyday Japanese, 花見 usually refers specifically to cherry-blossom viewing, even though it literally looks like flower viewing.

So most learners should understand:

• 花見の日 = the day of cherry-blossom viewing / a cherry-blossom viewing day

This cultural meaning is very common in Japanese.

Japanese chooses 来る or 行く based on the speaker’s point of view, just like English sometimes does, but not always in exactly the same way.

In this sentence, the park is treated as the relevant destination, and the sentence describes people coming to that place:

• 公園に来ます = come to the park

In another context, Japanese might use 行きます instead:

• 花見の日は、公園に大勢の人が行きます。

That would also be possible, but 来ます makes the movement feel directed toward the place being discussed. It is very natural when the park is the central location in the statement.