The Formal とき
際に is the written-register replacement for とき, used for occasions of some weight.
- ご利用の**際に**はお知らせください。(Please let us know when you use it.)
Only for Significant Occasions
This is the restriction that matters. とき covers anything; 際に covers events that are formal, procedural or notable:
| ○ 入国の際に | a procedure |
| ○ ご来店の際に | a formal visit |
| × トイレに行く際に | too mundane |
If the event is ordinary, use とき.
Tense Works Like とき
| Form | Timing |
|---|---|
| 使う際に | before or during the action |
| 使った際に | after the action is complete |
Where You Meet It
Notices, manuals, announcements, application forms, customer-service language. It is one of the highest-frequency patterns in written public Japanese, so recognition matters more than production.