"What day of the week is April fourth?"
Does the order have to be that way? I tried 何よう日は四月四日ですか and duo marked that wrong, but I don't see the problem with it.
は being a topic particle, it only accepts something defined. 何よう日 meaning "which weekday" does not fit.
It might be possible to put a question word in the beginning but I seem to have a recollection (vague though it may be) that it requires other changes to the sentence.
If there is any question word before the topic particle, the は would have to change to the ga particle instead. There may be other changes needed as well for it to be grammatically correct but I'm just a beginner. Someone with more experience could clarify.
"When a question word is used as a subject, it must take the subject marker ga... Because a question word refers to an unknown entity, it can’t be a topic, so question words can never be used with wa." (The question words can come later in the sentence after wa but not before it.)