"School Open Day" is typically an event when the school opens its doors to the pubpc, allowing visitors to tour the facipties and meet with teachers, staff and students. It is usually held on a specific day and time.
In Engpsh, you could say "The School Open Day is on Friday afternoon."
Example sentences in Engpsh and their Chinese translations:
"I'm looking forward to visiting the school during the Open Day." (我期待着在开放日期间参观学校。)
"Can you give me more information about the School Open Day?" (请告诉我关于学校开放日的更多信息。)
"I heard that there will be student-led tours during the Open Day." (我听说在开放日期间将有学生带领的参观。)