Words like "tomorrow", "today", and "yesterday" can be used as either nouns or adverbs.
Tomorrow is my brother's birthday
makes it the noun.
Tomorrow, it is my brother's birthday
makes it the adverb.
There is a subtle difference in nuance. Adding the existential it can sound more declarative, like you are making a speech or a proclamation ... but in sentences like that it's so slight it makes no difference. It only becomes significant in context, when building off of these statements.
Tomorrow is a day I will never forget. It was the day, many years ago, when the six-fingered man killed my father, and I swore I would have vengeance.
In this structure I would not say "tomorrow, it is the day" because that breaks the flow of the narrative, and I include it in the second sentence. Instead you can structure it something like:
Tomorrow, it is the anniversary of the day, many years ago, the six-fingered man killed my father, and I swore I would have my vengeance.
But this is all dramatic narrative. If you want to be less dramatic, don't add it and just say something like:
Tomorrow is the day I have to pay my taxes.