際に・折には: the difference
際にN2
Meaning
When, At the time of, In case of, On the occasion of
Past papers
Tested once in past papers
Example
炎天下で働く際には帽子などを持ってきてください。
When working under the scorching sun, remember to bring things like a hat.
折にはN1
Meaning
When, Occasion, Opportunity, Time
Past papers
Example
メール、本社の上司に：「その資料は直接お会いした折にお渡しします。」
Email to a boss at HQ: 'I will hand over that document when we meet in person.'
How to tell them apart
「折に／には」: gentle and refined, with a friendly 'taking this chance' feeling; used for planned visits or treasured past exchanges.
「際に／には」: more businesslike, like a manual or public notice, as in 「非常の際には」 (in an emergency).
「途端に」: stresses that something unexpected happened the instant an action occurred; it has nothing to do with a social opportunity.