"It is Sunday tomorrow."
Translation:Det er søndag i morgen.
It looks like 'i morgen' is a prepositional phrase and not a simple noun (or just one word like it is in english/german, like Morgen) like we are using it in other languages, think of 'i morgen' as meaning 'in the morning' and you will see why you cant elide the subject 'det' in the sentence.
That doesn't make much sense in English (why would Sunday not be the entire day?), but also, the Norwegian doesn't mean 'in the morning'.
i morgen = tomorrow
i morgen tidlig = tomorrow morning
om morgenen = in the morning (as in, 'jeg jobber om morgenen og studerer om ettermiddagen' - I work in the morning and study in the afternoon).