noun
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.
動物園の休園日は月曜日です。
The zoo's closing day is Monday.
休園日には入園できません。
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).