school day前面不要加冠词。但是，若是强调某一天时，可以加上冠词：It's a school day tomorrow, I can't stay up too late. 明天我还要上学，所以不能睡得太晚。

school可以用作名词

school的基本意思是“学校”,一般指儿童或中学生学习或受教育的场所,是可数名词,常与形容词性物主代词或定冠词连用。

school作“上学,学业”解时是不可数名词,其前不用冠词。

school还可表示大学里的“学院”。作“学派,流派”解时常用于习语a school of thought。

school用作名词的用法例句

She studies in a high school.她在一所中学学习。

Soon, the whole school knew about her win.不久，全校师生都知道她获胜了。

This is the argument of the school.这就是流行学派的论据。winter break: I'm going skiing during winter break. 我放寒假期间去滑雪。
Christmas: It's snowing on Christmas day. 圣诞节那天在下雪。
weekend: What are you doing next weekend? 你下周末打算干什么？
school day 前面也不需要：I can't play computer games on school days. 我上学时不能打电子游戏。