day on which a museum, library, etc. is closed; closed day
月曜日は美術館の休館日だ。
Monday is the museum's closing day.
休館日を調べずに来て中に入れなかった。
I didn't check the closing day and came only to find it shut.
祝日が休館日と重なって二日間閉まっていた。
A public holiday fell on the closing day, so it was shut for two days.