Here is a reference for you. It includes punctuation and capitalization. The capitalization is toward the bottom.
Since the planets are named after Greek gods, one could say the days of the week are named after Greek Gods in Spanish. In English, Tuesday and Wednesday do not have names which are connected with planets, but they do come from the names of gods - Germanic gods in their case. I suspect in both languages, it was the gods, not the astronomical bodies, which gave the days their names.
It may be redundant technically, but in normal English speech it's common (normally using it's rather than it is). There is no difference in meaning between Tomorrow it is Tuesday and Tomorrow is Tuesday.
I mean if you wanted to say "Tomorrow it is Tuesday" in Spanish, I can't think of any other way of saying it than "Mañana es martes".
It might be normal in speech but it does not seem common in formal writing. Searching in English books here https://books.google.com/ngrams It does not appear (as "it is" or the contraction). But without the "it" it does. Since duolingo wants us to translate articles for them they probably want us to use more formal English.
rocko your such a helper but with all respect: I can't read your posts seriously! Your profile picture is a Puppet from a awesome show (I hope you know that show because then why did you put that profile picture???) and the voice of the puppet is really funny sounding when he acts serious so I can't read your post without the puppets voice saying it instead and it makes me laugh so hard I'm sorry if this offends you I just wanted you to know that and instead of feeling upset about my thoughts feel happy that your helping but are also making your posts funny for me to read!
Yeah I totally agree, but in practice when we're trying to convert our own language into Spanish we're going to run through in our heads the way we'd say it in our native language first. So to say that "it is" cannot become "es" (or vice versa) in this scenario would be wrong in my opinion. As I mentioned, it may not be technically correct but it is definitely not wrong.
But as you say it's likely due to Duolingo's purpose/agenda rather than anything else.
I answered the previous question: “Today it is Monday” and it was correct, but "Tomorrow it is Tuesday" is incorrect! I believe that there is something I don’t remember well, but the point is that now I am trying to learn SPANISH not English. Duolingo's people must know that some users don't speak fluent English. We use Spanish -English course because there is not a course in our native language.
Side-note: Think days of week to their celestial counter parts: 1) lunes (the moon = "la luna") 2) martes (Mars = Marte) 3) miércoles (Mercury = Mercurio 4) jueves (Jupiter = Júpiter) 5) viernes (Venus = Venus) 6) sábados (Saturn = Saturno) 7) domingo (the sun = day of the Lord (Ltn. Org. "dies Dominca")