expression, adverb
on that occasion; at that time; in that case
Refers back to a previously mentioned situation or event, often used in formal or written contexts to mean 'when that happens' or 'in doing so'.
会議の日程が決まりました。その際、詳細な資料をお送りします。
The meeting date has been set. On that occasion, we will send you detailed materials.
パスワードを変更する場合は、その際に再ログインが必要です。
If you change your password, you will need to log in again at that time.
その時 is more general and conversational for 'at that time', while その際 is more formal and often implies a specific occasion or procedure.
Compound of その (that) and 際 (occasion, time).