際に and に際して: occasions for procedures and events
Read when an action is required in an instruction and actions taken on important occasions. Check noun and verb connections and relative timing without imposing an immediate before-or-after sequence.
At what point in the procedure?
申し込む際に、連絡先を記入してください。 asks applicants to enter contact details when applying. 際に formally identifies the occasion for the action without setting a precise clock time.
お帰りの際に、鍵を返してください。 asks people to return the key when leaving. Nouns connect with の際に, so do not mechanically use the same connection for both nouns and verbs.
An action on an important occasion
就職に際して、生活の予定を見直しました。 means reconsidering one's daily plans on the occasion of starting a job. に際して formally links such an important occasion with a related action.
A dictionary-form verb can connect too, as in 新しい施設を利用するに際して、注意点を確認してください。. This requests preparation for using a new facility without requiring it to occur only within a few minutes before the event.
Read timing across the whole sentence
窓口を訪れた際に、説明を受けました。 means receiving an explanation when visiting the service counter. The ta-form identifies that visit as the occasion; it does not require the entire visit to have ended before the explanation.
In everyday conversation, とき is natural in 帰るときに電話して. 際に and に際して suit notices or formal explanations, but a longer form does not automatically make every everyday sentence more courteous.
Quick check: Does 入学に際して説明会を開きます。 mean that the information session can take place only after enrollment is entirely over?
Show the answer and why
No. It announces a session on the occasion of entering the school. The exact date and whether it is before or after the entrance ceremony require further information.