〜折(に)
on the occasion, when (formal)
Verb plain / Noun + の + 折(に・には)
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折 is a formal, gracious "when, on the occasion of": 先日お会いした折に、その話を伺いました, "when I met you the other day, you told me about it".
It's typical of polite speech and letters. お越しの折には means "when you come this way", and 帰国の折には means "when you're back in the country".
Compare 際 (N3), a formal "when" used on notices and procedures: 地震の際は. 折 is warmer and more personal, and it's used for pleasant occasions, not emergencies.
In letters, 折 also appears in seasonal greetings: 寒さ厳しき折、どうぞご自愛ください, "in this bitterly cold season, please take care of yourself". The pattern is 〜の折から or 〜折.
Examples
近くにお越しの折には、ぜひお立ち寄りください。
When you're in the area, please do drop by.
先日お会いした折に、その話を伺いました。
When I met you the other day, you told me about it.
京都を訪れた折、古い友人に会った。
When I visited Kyoto, I met an old friend.
寒さ厳しき折、どうぞご自愛ください。
In this bitterly cold season, please take good care of yourself.
帰国の折には、ご連絡ください。
Please get in touch when you're back in the country.