the day in question; the appointed day; that very day (e.g., the day of an event or of issue/publication).
チケットは当日受付でお受け取りください。
Please pick up your tickets at the reception on the day of the event.
当日は混雑が予想されますので早めにお越しください。
Since congestion is expected on the day, please come early.
当日キャンセルは返金できません。
Cancellations on the day cannot be refunded.