I responded in a thread below, but to reiterate, these are the correct sentences: Tomorrow is Sunday. = Mañana es domingo. The movie is on Sunday. = La película es el domingo.
We use el and los when the word 'on' is used in the sentence, or when there is an ongoing occurrence: I go to the market on Mondays. = Voy al mercado los lunes (notice the use of 'on') I go swimming Tuesdays. = Voy a nadar los martes (ongoing occurrence)
In the case of "Tomorrow is Sunday," "Mañana es domingo" is correct because the word 'on' isn't in the sentence; you're simply stating it once.
If the sentence has an ongoing occurrence or uses the word 'on' (ex: "I go to the market on Sundays," means you go every Sunday), el or los (plural) is used.
We didn't make it wrong! Remember that on these discussion forums you are addressing your fellow learners. If you want help here regarding an answer you think was correct, but was marked wrong, the best thing to do is to tell the rest of us exactly what you put. Then we can either agree with you, that your answer was indeed correct, or else spot some small error that you missed which caused Duolingo to mark it wrong.