Perhaps, in context it wouldn't be: "What day is tomorrow?" "Tomorrow? It's Sunday." Or perhaps, "Tomorrow, it's Sunday." Technically, though, without something between "tomorrow" and "it's," it is incorrect because of the two subjects in one sentence. Some people do say that often though
You are correct, if there is a comma between "tomorrow" and "it's" in the sentence. If not, then both "tomorrow" and "it" act as subjects, which is counterintuitive since pronouns are supposed to replace nouns. Either way, in the Italian sentence, «domani» acts as the subject of the sentence and should be translated as such. If it said, «Domani, è domenica.» perhaps your translation of «Tomorrow, it's Sunday.» would be optimal. :D
Splitting hairs here, I know, but I still can't quite agree. In 'Domani è domenica', the subject - to my mind - is hidden in the word è. Domani is an adverbial phrase. In English, a comma would be indicated, but in the Romance languages I'm more familiar with (French and Spanish), it isn't, so I guess the same is true for Italian.. Actually, this is true for all the languages I'm somewhat familiar with, except English. (Of course, this doesn't matter very much. But as a linguaphile, I'm keen to get to the bottom of these things.)
Lol, I understand what you mean. I love languages, too! Then, I have a question for you. I don't know if this following situation will be comparable to the one which we are discussing, but in the Duolingo French courses for English speakers, it has sentences like « Ce cheval, est le mien. » and « Ce cochon, est le leur. ». In these cases, the subject too would be hidden in « est », so the phrase before the verb would act as an adjectival phrase, non? Sentences like these do differ from the same sentences without the comma, e.g. « Ce cheval est le mien. », as they have more of an emphatic nature, just as «Domani, è domenica.» or "Tomorrow, it's Sunday," have. Although, Duolingo does not really care much about punctuation, they do carry different meaning and, in cases where, when translating, an extra word could be added, in this case, the "it's," Duolingo does care about that. But I do think that the Romance languages would add the comma in there, if they wanted to emphasize it that is. And, you know what, languages do differ in how they classify certain words. I learned that the hard way in my class this past semester, European Languages: History and Theory. In English, for example, "some" is mostly an adjective, sometimes a pronoun or an adverb, but in Portuguese «algum» is always an indefinite pronoun. Maybe, it is one of these cases. Who knows? XD
One of the few grammar rules I'm still not sure about in English is whether the comma is required after an adverbial phrase at the beginning of the sentence. I don't know what the rule for this is in Italian either, but I learned in another comment thread that Italian doesn't use the serial comma, a.k.a. the "Oxford" comma (the comma used in English before the "and" in a list of three or more things).
English: "A cat, a dog, and a cow" Italian: "Un gatto, un cane e una mucca"
If the comma is not required after an adverbial phrase at the beginning of a sentence, then "D è d." (without the comma) seems ambiguous, but I would translate it by default as "T is S." because if the person writing it wanted to say "T, it's S.", then they probably would have made it clear by using the comma. Also, I think "T is S." is more common in English. (I am a native speaker of English.)
However, if the comma is required in Italian, then "T, it's S." could only be "D, è d.", and without the comma, it would have to be "T is S."