際 indicates an occasion or timing.
COMMON PATTERNS:
- 〜の際 (when ~, on the occasion of ~)
- 〜た際(は) (when [past event])
- この際 (at this point, while we're at it)
- いざという際 (in case of emergency)
FORMALITY:
- 際 is formal/written style
- 時 is the neutral equivalent
SIMILAR:
- 時: when (general)
- 際: occasion (formal)
- 場合: case, situation