際に・に際して: 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.
に際してN2
Meaning
On the occasion of, At the time of
Past papers
Example
大会への参加に際して、守ってほしいルールがあります。
There are some rules we would like you to follow when taking part in the tournament.
How to tell them apart
「際に」 is the all-purpose one: fine in everyday official notices, and it can follow the past form 「した際に」.
「に際して」 is for bigger occasions and ceremonies.
「にあたって」 looks forward to a new start, and is followed by preparations or reminders. It cannot be used for disasters or accidents.