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.
Next Friday is parents' observation day, so my mother will come to school.
On observation day, parents observe the classes.
parents' day; class observation day; open classroom day
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.