why 明天星期三， 为什么不加“是” 明天是星期三。who know it

both are correct.
With the sentence "明天星期三" which is a noun predicate sentence, noun as a predicate, mainly used for affirmative sentences, often used to talk about weather, time, holidays, full name, age, hometown ...
When adding "是", it turns into a verb predicate sentence

加不加是都聽得懂，口語可以不加。書面還是要加。

they are all right.
when someone ask "明天星期几？“ your answer is "明天星期三”
when they ask "明天是星期几？“ your answer " 明天是——”

Both are ok, But we usually say:"明天星期三"“是”is omitted.

Both are correct

中国人很少说明天是星期三。

both are correct :)

which one is right. i'm not sure