大多数不需要，但是,若是强调某一天时,可以加上冠词

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.

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.

But it'll be the weekend,sure you can have some fun.可是那是在周末,你难道就不能放松一下吗?