Some days of the week (Monday through Friday) just happen to end with the letter 's'...although they can be used in that form for both singular and plural: "Tengo clase el lunes próximo" (singular...one Monday in particular) or "Tengo clase todos los lunes" (plural...all of them). Both sábado and domingo are what you would expect: singular without a final 's', plural with an 's': el sábado/los sábados.
The rules for capitalization are different from English. While there is a long explanation for the rules, suffice to say, very few things are capitalized in Spanish when compared to English. Days of the week and months of the year are an example: lunes el 3 de abril. The only time they would be capitalized is when they are the first word in a sentence, which is a rule: the first word in a sentence is capitalized regardless of the part of speech or type of word it is.
So I just found this article about the definite article (no pun intended): http://spanish.about.com/od/adjectives/a/intro_def_art.htm which states this kind of sentences as an exception.
So what with the sentence "It's Monday tomorrow"? From my point of view it's correct sentence and then "Tomorrow it is Monday" is also correct. Tomorrow is not the subject in this sentence, it's just time and nothing more.
It´s Monday tomorrow? is not a correct question. In questions with the verb -to be the form is: to be + subject + complements. Is Monday tomorrow? http://www.engvid.com/grammar-making-to-be-questions/