折角【せっかく】

折角（せっかく） is an adverb that conveys the idea that something was done with special effort, represents a rare opportunity, or is otherwise precious and not to be wasted. It highlights the value of the effort or opportunity involved and often carries an emotional undertone – either gratitude and encouragement to take advantage of the situation, or disappointment when the effort goes to waste.

Encouraging someone to make the most of an opportunity

When paired with 〜～んだから or 〜～から, せっかく emphasizes that since effort was made or a rare chance has come, one should take full advantage of it.

Expressing regret when effort is wasted

When paired with 〜～のに or 〜～けど, せっかく expresses disappointment that something done with effort or a special opportunity ended up being in vain.

せっかくの + Noun

せっかく can directly modify a noun with の to mean "precious," "hard-won," or "long-awaited."

Key nuances

• せっかく always implies that effort was involved or the situation is special/rare. It would sound unnatural for trivial, everyday actions.
• It often carries an implicit suggestion: "It would be a waste not to…" or "What a shame that…"
• It is used in both polite and casual speech.