sekkaku sakura ga mankai nanoni, kyou ha ame de hanami ga dekimasen.

せっかく adds the feeling of what a waste or even though this was a good/special opportunity.

In this sentence, it suggests something like:

• The cherry blossoms are finally in full bloom, and yet...
• It’s such a shame, because the timing is perfect, but...

So it is not just factual. It shows regret or disappointment.

満開 means full bloom.

It is treated like a noun or na-adjective type word in Japanese. That is why you get:

• 桜が満開です = The cherry blossoms are in full bloom.
• 桜が満開なのに = Even though the cherry blossoms are in full bloom...

So it does not behave like an i-adjective such as 高い.

Both can be possible in some contexts, but 桜が満開 is the more natural way to present the state of the cherry blossoms.

Here, が marks 桜 as the thing that is in the state of full bloom.

• 桜が満開 = The cherry blossoms are in full bloom.
• 桜は満開 would sound more contrastive, like As for the cherry blossoms, they are in full bloom.

So が is more neutral and natural here.

Because 満開 is a noun/na-adjective type word, and before のに it needs な.

So:

• 静かなのに = even though it is quiet
• 便利なのに = even though it is convenient
• 満開なのに = even though they are in full bloom

With an i-adjective, you do not use な:

• 高いのに
• 寒いのに

So 満開のに is incorrect.

なのに means even though, despite the fact that, or and yet.

But it often carries an emotional nuance of:

• unexpected contrast
• frustration
• disappointment

In this sentence, the speaker feels something like:

• The blossoms are perfectly timed, and yet it’s raining.
• What bad luck.

So なのに is stronger and more emotional than a simple but.

は marks 今日 as the topic: as for today.

So the sentence frames the situation like this:

• As for today, it’s rainy, so...

Using は also gives a slight contrastive feeling, as if implying:

• Today is rainy, unfortunately.
• Maybe other days were or will be different.

今日が would sound less natural here unless you were specifically identifying today as the day in some special contrast or answer to a question.

Here, で is not the location particle. It shows cause/reason or circumstance.

So 雨で means:

• because of the rain
• due to the rain

This use of で is common with things like weather, illness, or other conditions:

• 風邪で休みました = I was absent because of a cold.
• 雪で電車が止まった = The train stopped because of snow.

So in this sentence, 雨で means the rain is the reason hanami cannot be done.

Because できる usually takes が, not を, for the thing that is possible.

So:

• 花見ができる = can do hanami
• 日本語が話せる = can speak Japanese
• ピアノが弾ける = can play piano

This is a very common pattern with potential expressions in Japanese.

Since 花見する is a verbal-noun expression, when it becomes a potential idea, it naturally becomes:

• 花見ができる
• 花見ができません

Because the point is inability, not choice.

• 花見をしません = We/I will not do hanami.
  This could sound like a decision or refusal.
• 花見ができません = We/I cannot do hanami.
  This shows that circumstances make it impossible.

Since the reason is rain, できません is the natural choice.

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

It often means more than just looking at flowers. It can include:

• going to a park
• walking under the blossoms
• having a picnic
• eating and drinking with friends, family, or coworkers

So in this sentence, 花見 refers to the usual spring activity of enjoying the cherry blossoms.

Japanese nouns usually do not show singular/plural the way English does.

So 桜 can mean:

• a cherry blossom
• cherry blossoms
• cherry trees

In this sentence, from context, it clearly means the cherry blossoms in general.

English has to choose singular or plural, but Japanese often leaves that unstated unless it really matters.