Examples:
- 面接が終わって退室する際は、会釈をし、静かにドアを閉めましょう。When you leave the room after the interview, give a bow and close the door quietly.
- ご乗車の際には、乗務員に乗車券をご提示ください。Please present your ticket to the crew when boarding.
- 先月帰省した際に、高校の先生を訪ねた。When I returned home last month, I visited a high school teacher.
It is equal to "~とき", but it is a written word with the meaning of "timing""opportunity".