sekkaku kyouto made kitano ni, hakubutukan ga yasumi de sukosi zannen datta.

せっかく adds the idea of making a special effort or going to the trouble of doing something.

So in this sentence, it suggests something like:

• after coming all the way to Kyoto
• since I made the effort to come to Kyoto

It often appears when the result is disappointing or does not match expectations, which fits well with のに later in the sentence.

In 京都まで来た, まで means as far as or all the way to.

It emphasizes the distance or effort involved:

• 京都に来た = came to Kyoto
• 京都まで来た = came all the way to Kyoto

So まで helps support the feeling created by せっかく: the speaker made a real effort to get there.

In this sentence, it is best to treat のに as a single grammar pattern attached to the plain form of the verb:

• 来た + のに

Here, のに means even though, although, or despite the fact that.

So this is not the explanatory の followed separately by に. It is the set expression のに.

のに shows a result that goes against expectations and often carries a feeling of regret, complaint, or disappointment.

So the flow is:

• I came all the way to Kyoto
• but contrary to what I hoped, the museum was closed

Compared with something softer like けど, のに sounds more emotionally invested. It fits very naturally with 残念だった.

が marks 博物館 as the subject of the state 休み:

• 博物館が休み = the museum was closed

Using が here is neutral and straightforward.

If you said 博物館は休み, it would sound more like:

• as for the museum, it was closed
• possibly with some contrast, such as implying that something else was open

So が is a very natural choice when simply stating the reason for the disappointment.

Here, 休み behaves like a noun / na-adjective-type predicate, and で is its connective form.

So:

• 休みだ = is closed / is a holiday / is off
• 休みで = being closed / because it was closed

In this sentence, で connects the clause to what follows and also gives a reason or circumstance:

• 博物館が休みで、少し残念だった
• The museum was closed, so it was a little disappointing

This is similar to how other noun-like or na-adjective predicates connect:

• 静かで
• 便利で

休み literally relates to rest, day off, or holiday, but when used about a store, museum, restaurant, and so on, it commonly means that the place is closed.

So:

• 店が休み = the shop is closed
• 博物館が休み = the museum is closed

This is very common Japanese usage.

少し means a little, and here it softens the statement.

Japanese often avoids sounding too strong or dramatic unless the speaker really wants that effect. So:

• 残念だった = it was disappointing
• 少し残念だった = it was a little disappointing

This can sound more natural, modest, or understated.

残念 is a na-adjective-type word, so in plain past form it becomes:

• 残念だった = was disappointing

This is not a verb by itself. The past meaning comes from だった, the plain past form of だ.

A polite version would be:

• 残念でした

So the sentence is in plain style.

The speaker is describing a past experience:

• they came to Kyoto
• the museum was closed
• they felt disappointed

That is why the final predicate is 残念だった.

Even though the disappointment may still be emotionally true now, the sentence presents it as part of a past event or narration.

Japanese often omits subjects when they are clear from context.

In this sentence, the unstated subject is most naturally the speaker:

• (I) came all the way to Kyoto
• (I) felt a little disappointed

You usually do not need to say 私は unless you want contrast, emphasis, or extra clarity.