〜際（さい）

About

～際さい can attach to a present/past casual verb, or a noun using の, and is used to express "when" or "on the occasion of".

～際さい is most commonly used in polite/formal spoken situations, or in written contexts.

〜際さい may appear on its own (written), or with は, に, or には attached. It may also attach to a following noun using の.

〜際さい has a nuance of officiality to it, and isn't natural to use with unimportant or mundane actions.

E.g. 免許証めんきょしょうを更新こうしんする際さいに、こちらの書類しょるいをお持もちください。 "When renewing your license, please bring these forms."

E.g. お帰かえりの際さいは、お忘わすれ物ものにご注意ちゅういください。 "When leaving, please be sure not to forget any belongings."

The phrase,「この際さい」 means something like "since we have a chance..." or "now is a good time to do..." (lit. "this opportunity"), and is used much more colloquially than 際さい typically is:

E.g. この際さい、余あまっている野菜やさいは全部ぜんぶ入いれちゃおうよ。 "We may as well put in all of the leftover veggies."

E.g. この際さいだからはっきり言うけど、部屋へやの掃除そうじをして。 "I'll take this occasion to say it clearly: clean your room."

Summary

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  ～際さい is used to express "when" or "on the occasion of"

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  ～際さい is most commonly used in polite/formal spoken situations, or in written contexts