の際は

when

→ の
phrase Rare #29,887
when A formal expression used for an occasion or time; often appears in polite announcements and requests.
JA: お帰りの際は、受付にお声がけください。
EN: When you leave, please let the reception desk know.

こちらにおいでの際は必ずお寄りください。

Please be sure to drop by whenever you’re in the area.

仕事の際は、常にネクタイを着用しますか?

Do you always wear a tie to work?

ご移転の際はお知らせ下さい。

Please let us know if you relocate.

今後旅行の際はぜひ当社をご検討ください。

Please consider our company for your future travel needs.

お帰りの際は傘をお忘れなく。

Don't forget your umbrella when you leave.

ご用の際はお呼び下さい

Please call me if you need anything.

— レベッカ (1940)