〜際(に)
when, at the time of (formal)
Verb plain form / Noun + の + 際(に) · 際は
Sign up free to review this point on a spaced schedule.
際 is a formal word for "occasion", and it works like とき: お帰りの際は、忘れ物にご注意ください, "when leaving, please take care not to forget your belongings".
You'll hear it in announcements, read it in instructions and notices, and use it in business emails: 日本に来た際には、ぜひご連絡ください, "when you come to Japan, do get in touch".
After a noun it takes の; after a verb it follows the plain form. The に is optional, and 際は picks out the occasion as a topic: 地震の際は, "in the event of an earthquake".
In everyday conversation, とき is the natural choice. Using 際 with friends sounds stiff.
Examples
お帰りの際は、忘れ物にご注意ください。
When leaving, please take care not to forget your belongings.
入場の際、チケットを見せてください。
Please show your ticket on entry.
日本に来た際には、ぜひ連絡してください。
When you come to Japan, please do get in touch.
地震の際は、エレベーターを使わないでください。
In the event of an earthquake, do not use the lifts.
会議の際に、資料を配ります。
We'll hand out the documents at the meeting.