有上学日子, 有游戏日子. same meaning as: time for study, time for play (有的时间学习，有娱乐的时间)
on school day，例如：
1.On school day, what is the maximum speed through school zones in the city town or villages?在城市内上学日，在穿过校区地带时，最高的时速是多少？
2.Ironed shirts on a school day.在上学的日子穿熨烫整洁的衬衣。
3.On the school opening day, I rose up very early, earlier than usual, because I was greatly excited to see my new school.开学那一天，我很早起身，比平常时还早，因为我很兴奋的要看看我的新学校。
如果是说在学校的某一天，用：on a school day
如果表达通常的情况，用：on school days
应该是on school day，
因为在英语里，day前面加on。