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.