"Demain nous sommes samedi."
What is the literal translation / reason why this works? Is it "tomorrow we are at Saturday"?
In French, you usually translate "It is (+ week day)" into "Nous sommes". For example: "It is Saturday" translates into "Nous sommes samedi." (or "C'est samedi.")
it's because "tomorrow we are at Saturday" is not natural English. However, at other times, Duolingo rejects "natural English" and insists on a more literal translation. So it gets very frustrating, I agree.
According to a podcast I listen to (Francais Authentique) 'On est samedi' is how the french would say 'it's Saturday' in everyday conversation.