In the UK, this would be referred to as a parent day or a school open day. A 'parent day' would normally include one or more parents coming into the school to discuss the child's progress and have a look at some completed work of the child. It would not usually include actually watching the child work due to the limited space of classrooms. A school 'open day' may be something a little different which would normally be geared towards showing parents of prospective students what the school is like. During such days, students are not normally on the campus, or if they are, their lessons would not be interrupted. These days, safety concerns with bringing adults onto the campus, is a high priority and the 'openness' of any 'open' day is decreasing.
イギリスでは、参観日と似た日はparent day やschool open dayに当たるでしょう。
'parent day' では親の一方または両親が子供の学校へ出向き子供の成長ぶりを話したり、子供が完成させた作品などを見ます。
クラスのスペースなどの都合で実際の授業参観は通常ありません。
学校の 'open day'（オープンキャンパス）は少し違い、将来通うかもしれない生徒の親御さんが学校の様子を見に来る日です。
通常その日は生徒がキャンパスに居ません。
仮に生徒の授業をあった場合は、授業の中断や邪魔はありません。
近年では、一般公開で大人をキャンパスに入れる事の安全性が問われ、一般に開放することが少なくなってきています。
Observing is watching something or someone doing what they normally do. Observation,open,or parents day are all allowing you the chance to visit your child’s school and watch him/her do what they normally do.
School open day a is day when teachers visit a school to which they do not usually have access to. Therefore on this day they are allowed to come and watch their children learn which doesn't happen on any other day.
"School open day"とは、学校の公開日の事です。通常は他校の先生が行くことのない学校を尋ねることが出来ます。ですので、その日は他の日にはできないような、子供たちの授業などを参観することが出来ます。
Sometimes schools in the UK allow people
to visit during an Open Day.
This is when students accompany their parents
and look around the school and talk to teachers
about what has been achieved during the school year.
Sometimes when children go to school they also
have Parents Evening. This is when the student's
progress is discussed with the class teachers.
他のアンカーの方も回答してらっしゃるように、「授業参観日」は
school open dayと言えば良いでしょう。
例）
When is the next open school day?
「次の授業参観日はいつですか？」
以上です。
少しでも参考になれば幸いです(*^_^*)
★★大阪カフェレッスン英会話講師KOGACHI
Parent day example sentence: I was excited to see so many friendly parents at my school on parent day yesterday.
School open day example sentence: It is my school open day tomorrow and many of the parents are invited to see what kind of environment their children learn in.
Observation day example sentence: I am always nervous when it comes to observation day because my parents are very strict and they will ask the teacher many questions regarding my behavior.
アメリカでは open house と表現することができます。
しかし、厳密には日本の「授業参観」とまったく同じではありません。
open house は通常は入れない場所（学校など）が特別に開放するイベントなどを意味します。
私が育った地域では毎年、小学校でもこのような日があり、親や保護者が学校の中にアクセスし子供達の作品などを見ることができました。
例：
Are you coming to the open house?
オープンハウスには来ますか？
お役に立てればうれしいです。
日本 
アメリカ合衆国 
イギリス 
南アフリカ 
