I don't understand what regulates the usage of IT in the following sentences. My statement is based on opinions of natives. For example, they say:
1) What day is it today? - OK
2) Today, it is Monday. - wrong (should be - It is Monday, today.)
I think people consider TODAY a noun, if so, then we really have "NOUN + IT" which is wrong. But TODAY can be an adverbial modifier of time.
3) Today is Tom's birthday - OK
4) Whose birthday is today? wrong (should be - Whose birthday is it today?)
If 3 is correct, why is 4 wrong?
5) Today it is cold. - OK
6) Today it is Sunday. - wrong
Any reliable explanation?