This sentence seems to be right -- I have a second-lang Spanish speaker who says it sounds better to him than ¿Cuál día es mañana? and a Google search turns up a lot of Qué results when I search for the question using Cuál...
However, I am curious as to whether anyone can explain this. It seems like a classic case of a cuál-what instead of a qué-what.
My Spanish teachers have told me there are situations when the cual/que choice is no big deal. Que/cual dia? could be answered with "lunes" (one of several options, cual) or "the day I try to pass my driving test" or "my birthday" or "the day after this one" (description/definition, que).
Using "cuál" before a noun is wrong!
Have a look to this link: http://www.onetoonespanish.co.uk/blog/the-difference-between-que-and-cual-in-spanish.htm
When the question implies a choice between two or more alternatives and conveys the idea of selection, ¿Cuál + verb/preposition? or ¿Qué + noun? can be used.
In this case, although cuál can be used as "what," this is more of a "what day of many/which day of the week" is tomorrow, sorta question. At least, its how i perceive it. Por ejemplo, hoy es lunes.
However, the "Qué" could be used in the sense of, "What day (assuming something significant is supposed to happen) is tomorrow?" Por ejemplo, mañana es mi cumpleaños.
Could a native verify/correct me if I am wrong? Thanks! ^_^
I suspect that Wowpow2300 is uncomfortable with the phrase "native speaker of." Some people find this phrasing to be an offensive pejorative. I personally find this phrasing informative and acceptable because it's much quicker to say "a native English speaker" than to say "a person whose mother tongue is English."
I believe you may have been marked wrong because the usual syntactical order of this question is adverb (what), predicate verb (is), subject of sentence (it), subject complement (tomorrow), which is "What is the day tomorrow?
English reverses the S–V (subject/verb) order of a sentence statement to V–S (verb/subject) order when the sentence is a question.