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day in question; appointed day; that very day; the day (of issue, publication, etc.)
当日は朝早くから準備を始めることになる。
We started preparing early in the morning that day.
チケットは当日券のみ販売しています。
Tickets are only sold on the day.
試験当日は緊張してよく眠れなかった。
I couldn't sleep from nervousness on exam day.