折角【せっかく】

折角 marks an effort someone made or a chance that doesn't come often, something too good to waste. It is almost always written in hiragana, せっかく. The verb after it is usually in the past plain form, and what follows says either "so make the most of it" or "and it went to waste".

With から or んだから, the second half is what you should do so the effort isn't wasted: a suggestion, a request or an intention.

せっかくですから on its own politely accepts an offer: "since you've been so kind".

When the effort is wasted

With のに or けど, the effort or the chance came to nothing, and the sentence sounds disappointed.

せっかくの + noun

With の, せっかく describes a noun: a precious day off, a rare chance, a long-awaited trip.

せっかく or わざわざ

わざわざ says someone went out of their way, and it can stand on its own in thanks. せっかく always looks ahead to whether the effort pays off, so it can't simply thank someone.

• × せっかく来てくれてありがとう → わざわざ来てくれてありがとう