cooperation day; parent cooperation day
A day designated by a nursery school or similar institution when parents are asked to keep their children at home, often to reduce staff workload or for special events.
Next Friday is a cooperation day, so please look after your child at home.
Compound of 協力 (きょうりょく, 'cooperation') and 日 (び, 'day'). The term is used in the context of childcare facilities.