In the England where I was born and raised, "It is" or "it's" would be completly acceptable and more usual. Usually, in conversation, it would be spoken reversed:-
"Is it Tuesday today?"
"No. It's Tuesday tomorrow".
or/ "What day is it?". "It's Tuesday". In fact I can't imagine anyone except a foreigner saying "Tomorrow IS Tuesday".
"it" refers to the day. "Tuesday is the *name" of the day. Technically, Tomorrow, IT WILL BE Tuesday. (future tense - but don't sweat it :-) )
Your English is good. Thanks for explaining. It reminds me of when we lived in Campinas. Each day there was a farmers' market in a different neighborhood. On Fridays the farmer's market was in a park 2 blocks from where we lived. When I learned about these farmers' markets, the days of the week finally made sense to me. The 6th fair of the week was in my neighborhood.
So Portuguese and Brazilians seem to count the days of the week starting on Sunday. It's funny because for me Monday would be the first day of the week :-) In Russian, the days of the week are also derived from numbers, but only Tuesday, Thursday and Friday. In the Germanic and Romance languages they are mainly derived from old Germanic resp. Roman gods.
This system comes from medieval ecclesiastical Latin in order to avoid using names of ancient gods. For example: Ash Wednesday is Feria Quarta Cinerum. (A sad date for me.) Interesting: why the also 'very catholic' other romance speaking countries have kept the old concept. More: https://en.m.wikipedia.org/wiki/Feria