syuumatu ha kazoku to issyo ni kouen de hanami wo simasu.

週末 is a time expression, and many common time words in Japanese can appear without に.

Here, は marks 週末 as the topic:
週末は ... します = As for the weekend, ... do ... / On the weekend, ...

This can also give a slight contrastive feel, like when it comes to the weekend.

A few useful comparisons:

• 週末は花見をします。 = As for the weekend / On weekends, I do flower viewing.
• 週末に花見をします。 = I will do flower viewing this weekend / on the weekend.

Both can be natural, but は focuses on weekend as the topic, while に simply marks the time.

Japanese often leaves out the subject when it is clear from context.

So instead of saying:

• 私は週末は家族と一緒に公園で花見をします。

Japanese very often just says:

• 週末は家族と一緒に公園で花見をします。

The listener usually understands who is being talked about from the situation. This is extremely common in Japanese and is one of the first things English speakers notice.

家族と一緒に means together with my/the family.

It breaks down like this:

• 家族 = family
• と = with
• 一緒に = together

So:

• 家族と = with family
• 家族と一緒に = together with family

Using both is very natural. 一緒に emphasizes the idea of doing it together.

You can sometimes omit 一緒に:

• 家族と花見をします。

This still means I do flower viewing with my family, but 家族と一緒に sounds a little more explicitly like together with my family.

In 一緒に, the に turns 一緒 into an adverb-like expression.

So 一緒に means together in the sense of doing something together.

Examples:

• 友だちと一緒に行きます。 = I go together with my friend.
• 家族と一緒に食べます。 = I eat together with my family.

You should learn 一緒に as a set phrase.

で marks the place where an action happens.

Since 花見をします is an activity, the park is the location where that activity takes place:

• 公園で花見をします。 = do flower viewing at the park

Compare:

• 公園に行きます。 = go to the park
  
  • に marks destination
• 公園で遊びます。 = play at the park
  
  • で marks location of action

So in this sentence, で is correct because the action します happens in the park.

花見 is a noun, and する is used to turn many nouns into actions.

So:

• 花見 = flower viewing / cherry-blossom viewing
• 花見をする = to do flower viewing

In careful or standard grammar, 花見をします is the full form.

In casual speech, Japanese sometimes drops を with する nouns:

• 花見する

That is common in conversation, but 花見をします is the safer form for learners and is more standard in textbook-style Japanese.

Literally, 花見 means flower viewing, but in Japanese it usually refers specifically to cherry-blossom viewing.

In real life, 花見 often means going to a park, seeing the cherry blossoms, and sometimes eating, drinking, or spending time with family or friends there.

You may also see:

• お花見

The お makes it sound a little more polite or softer.

It can mean either one, depending on context.

Japanese non-past forms like します can refer to:

• a future action
• a habitual action
• a general statement

So:

• 週末は家族と一緒に公園で花見をします。

could mean:

• This weekend, I’ll go flower viewing in the park with my family.
• On weekends, I go flower viewing in the park with my family.

In real conversation, context usually makes it clear which meaning is intended.

Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

The basic idea here is:

• 週末は = topic/time
• 家族と一緒に = with family / together
• 公園で = at the park
• 花見をします = do flower viewing

You could rearrange some parts and still be understood, for example:

• 週末は公園で家族と一緒に花見をします。

This sounds natural too.

What usually stays at the end is the verb phrase:

• 花見をします

That is one of the most important word-order rules in Japanese: the verb typically comes near the end.

In this sentence, 家族 would normally be understood as my family or one’s family, depending on who is speaking.

Japanese often does not use words like my when the meaning is obvious from context.

So:

• 家族と一緒に usually means with my family
• not necessarily with the family in a general sense

This is another common difference from English: Japanese often leaves possession unstated when it is already clear.

一緒 originally has the sense of the same or together, and in everyday Japanese learners most often meet it in patterns like:

• 〜と一緒に = together with ~
• 一緒に〜する = do ~ together

So in this sentence, it functions as part of the expression 家族と一緒に, meaning together with family.

You do not need to worry too much about labeling it at this stage. The most useful thing is to memorize the pattern:

• person + と一緒に + verb

Examples:

• 友だちと一緒に勉強します。
• 母と一緒に買い物します。

Only if you are talking about going to the park.

This sentence is about doing flower viewing at the park, so 公園で is the natural choice.

If you wanted to include the idea of movement, you could say something like:

• 週末は家族と一緒に公園に行って、花見をします。
  = This weekend, I’ll go to the park with my family and do flower viewing.

So:

• 公園に / 公園へ = to the park
• 公園で = at the park

The original sentence focuses on the activity location, not the movement.