Why is the auxiliary do sometimes omitted in questions?
'What day really starts a new week?'
But in what cases is it unnecessary?
***** NOT A TEACHER *****
Where do you study?
Why does Tom have blue hair?
How does one cook a lobster?
When did the first newspaper start?
What day really [ = in reality] starts a new week?
I think that you have now answered your own question. Am I right? You probably said that "what" is being used as an
adjective to modify the noun "day." Therefore, it is not necessary to use a form of "do" for the question.
What shirt should I wear today?
What grammar helpline is your favorite one?
What newspaper do you read? (You do read what newspaper?)
What day did he visit? (He did visit [on] what day?)
When the question word, who, what, which ...is the subject of the verb, we don't need DO.
Who works here? I do/I work here.
Who do you want to see? I want to see Mr Jones.
What days suit you best? Monday and Thursday (suit me best).
What days do you prefer? I prefer Mondays and Thursdays.