Why the noun 'heute' is not capitalized?
I know that in German, What day is it today? is Welcher Tag ist heute?
Then why I should not write Heute instead of heute? Maybe heute is an adverb rather than a noun here, so the sentence is equivalent to
'Welcher Tag ist es heute?'? and the pronoun 'es' is omitted?

Question

Why the noun 'heute' is not capitalized?
I know that in German, What day is it today? is Welcher Tag ist heute? 
Then why I should not write Heute instead of heute? Maybe heute is an adverb rather than a noun here, so the sentence is equivalent to 
'Welcher Tag ist es heute?'? and the pronoun 'es' is omitted?

Answer

Hello,

You are right. Heute is an adverb in the question: Welcher Tag ist heute? 
Unlike in English, we don't need the pronoun es here, just leave it out.

Answer

it's an adverb of time