asita no hanabi no koto wo kangaeru dake de, wakuwakusimasu.

In this sentence, 明日 means tomorrow, and 明日の花火 means tomorrow’s fireworks or the fireworks tomorrow.

Here, の links nouns together, like ’s or of in English:

• 明日 = tomorrow
• 花火 = fireworks
• 明日の花火 = tomorrow’s fireworks

So 明日の花火 acts as one noun phrase.

This の is also linking nouns, but the pattern X のこと has a special meaning: about X or the matter of X.

So:

• 花火のこと = about the fireworks / the fireworks as a topic

This is a very common pattern in Japanese:

• 彼のことを知っています。 = I know about him / I know him.
• 日本のことをもっと勉強したい。 = I want to study more about Japan.

In your sentence, 明日の花火のことを考える means to think about tomorrow’s fireworks.

Because 考える usually takes something as a topic/content of thought, and X のことを考える is a natural way to say think about X.

So:

• 花火のことを考える = think about the fireworks

If you said 花火を考える, it would sound unnatural in most normal situations. Japanese often uses こと to turn something into the matter of..., which works well with verbs like 考える.

The particle を marks what is being thought about in this expression.

In 花火のことを考える:

• 花火のこと = the fireworks / the matter of the fireworks
• を = object marker
• 考える = think

So the whole chunk means to think about tomorrow’s fireworks.

Even though English uses about, Japanese often uses を with this set phrase.

Because 考える is modifying だけ.

The pattern is:

• dictionary form + だけで = just by doing..., only by doing..., simply doing... is enough to...

So:

• 考えるだけで = just by thinking about it / simply thinking about it

This is a fixed and very common structure. The plain form is used before だけ.

Examples:

• 見るだけで楽しい。 = Just looking at it is fun.
• 名前を聞くだけで怖い。 = Just hearing the name is scary.

だけ means only or just, and で here gives the sense of by / with / because of that alone.

So 考えるだけで、わくわくします means:

• Just thinking about it makes me excited
• Simply thinking about it is enough to make me excited

The idea is that no actual fireworks are needed yet — merely thinking about them already causes excitement.

The comma separates the condition/cause-like part from the main result:

• 明日の花火のことを考えるだけで、 = just by thinking about tomorrow’s fireworks,
• わくわくします。 = I get excited.

Japanese commas are often more flexible than English commas, so this comma is mainly there to make the sentence easier to read.

わくわくする means to feel excited, thrilled, or full of eager anticipation.

It usually has a positive feeling, like being excited for something fun:

• a trip
• a festival
• fireworks
• a present

So わくわくします is a polite form meaning I get excited or I feel excited.

Because わくわく is an onomatopoeic or mimetic expression, and these are very often written in hiragana or katakana rather than kanji.

In Japanese, words like these often express feelings, sounds, or manner:

• どきどき = heartbeat / nervous excitement
• わくわく = excited anticipation
• きらきら = sparkling

So わくわくする is a very natural everyday expression.

The subject is omitted because Japanese often leaves out information that is clear from context.

In this sentence, わくわくします naturally suggests that the speaker is the one feeling excited:

• (I) get excited just thinking about tomorrow’s fireworks.

Japanese frequently omits pronouns like I, you, and he/she when they are understood.

It is talking about a present/general feeling in relation to a future event.

• 明日の花火 = tomorrow’s fireworks
• 考えるだけで = just by thinking about them
• わくわくします = I feel excited / I get excited

So the fireworks are in the future, but the excitement is happening now whenever the speaker thinks about them.

Yes. That is a very natural translation.

Other natural translations include:

• Just thinking about tomorrow’s fireworks makes me excited.
• I get excited just thinking about tomorrow’s fireworks.
• Simply thinking about tomorrow’s fireworks makes me feel excited.

They all capture the same basic meaning. The Japanese sentence does not force one exact English wording.

Japanese nouns usually do not change form for singular or plural.

So 花火 can mean:

• a firework
• fireworks
• a fireworks display

In this sentence, it most naturally refers to the fireworks event/display tomorrow, so English usually translates it as fireworks.

It can mean either, depending on how natural the English translation needs to be.

Literally, it is tomorrow’s fireworks. But in natural English, people often say:

• the fireworks tomorrow
• tomorrow’s fireworks
• the fireworks display tomorrow

All are fine. Japanese 明日の花火 simply identifies the fireworks associated with tomorrow.