～際(に)— On The Occasion Of

• お降りの際は、足元にご注意ください。

  Oori no sai wa, ashimoto ni gochuui kudasai.

  When getting off, please watch your step.

  お降りの + 際 = polite 'on the occasion of alighting'

• ご来店の際には、会員証をご提示ください。

  Goraiten no sai ni wa, kaiinshou wo goteiji kudasai.

  When visiting the store, please present your membership card.

  ご来店の + 際に = formal 'at the time of your visit'

• 非常の際は、このボタンを押してください。

  Hijou no sai wa, kono botan wo oshite kudasai.

  In case of emergency, please press this button.

  非常の + 際 = 'on the occasion of an emergency'

• ・Register: formal/written; used in notices, announcements, and business/keigo contexts.
• ・Attaches to nouns via の, and to verbs in dictionary or た-form.
• ・For casual everyday situations, use とき instead.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～際(に) is tested against.

• ～ないうちに — Before It Happens
• ～上(で) — After Doing / In the Process Of
• ～から～にかけて — Spanning From To
• 最中に — Right in the Thick of It
• 次第 — The Moment It's Done
• ～て以来 — Ever Since Then
• ～ところに／～ところへ — Right At That Moment
• ～ところを — Caught In The Act