closing day (of a park, zoo, kindergarten, etc.)
Refers to a regular day when a facility such as a park, zoo, or kindergarten is closed to the public. Often seen on signs or schedules.
The zoo's closing day is Monday.
You cannot enter the park on its closing day.
休館日 is a closing day for a building or facility like a museum or library, whereas 休園日 is for a park or garden.
Compound of 休 (rest, holiday), 園 (park, garden), and 日 (day).