際|さいsai[1]atamadaka (head-high)

1. nounoccasion; time; circumstances; (in) case (of); when
   火事かじの際さいは１１９番ばんに電話でんわしてください。↗

   In the case of fire, dial 119.

• Synonym時ときNeutralThe neutral, everyday default for 'time; when'; 「際」 is more formal and typical of notices and announcements, as in 「ご利用の際は」.
• Synonym場合ばあいNeutralA neutral word for a situation or case, often with a conditional feel, as in 「雨の場合」; 「際」 marks when something takes place and can also cover cases yet to happen.
• Synonym折おりWrittenWritten/formal prose for an occasion or opportunity, heard mostly in letters, as in 「お近くにお越しの折は」; the tone is stately and courteous.

For everyday 'when', 「時」 is the safe choice. 「際」 suits notices and formal occasions and works for both actual and hypothetical situations. Use 「場合」 when the focus is on conditions or circumstances, and 「折」 for occasions in letters.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. （サイ）とき。場合ばあい。
2. （サイ）（「この際さい」などのかたちで）ちょうどいい機会きかい。また副詞ふくし的てきに、思い切おもいきって、いっそ。