Please don't hate me if I'm wrong, but Sabbath is religious, right? So, for Christians such as myself, I think that would be like saying "Tomorrow is Mass" instead of "Tomorrow is Sunday." Because one is a day of the week, and the other is a religious day. If I'm wrong, I'm SO sorry! :)
I don't think the it in "Tomorrow it is Saturday" is gramaticly correct.
Small letters for the days, months or seasons, languages and nacionalities (gentilicios). For instance..."Yo soy un mexicano que habla español durante la semana e inglés el domingo".
Capital letters on the start and special days, historical dates and countries. And only the first letter of the title is capitalized in a novel..."Cien años de soledad", no "One Hundred Years of Solitude" as in English.