An adverbial expression meaning 'on the occasion of; at the time when'—used to indicate when something happens, often following a noun.
来日する際に、ぜひ連絡してください。
Please be sure to contact me when you come to Japan.
契約を解除する際には、書面で通知する必要がある。
When terminating the contract, it is necessary to notify in writing.
搬入の際に壊れ物がないか確認しましたか？
Did you check for any fragile items when moving them in?