noun
parents' observation day; open day at school
A designated day when parents visit their child's school to observe classes and school life. Common in Japanese elementary and junior high schools.
来週の金曜日は参観日なので、母が学校に来ます。
Next Friday is parents' observation day, so my mother will come to school.
参観日には、保護者が授業の様子
On observation day, parents observe the classes.
A home visit by the teacher, not a school event for parents.
Compound of 参観 (visit for observation) and 日 (day). The exact historical derivation is uncertain, but the term is widely used in modern Japanese school contexts.