"Tomorrow is Saturday, not Sunday."
Translation:Jutro jest sobota, nie niedziela.
I also have been wondering that with some sentences. But with this one, keep in mind that 'jutro' is always an adverb in Polish, not a noun, so literally it would be more like 'It is Saturday today,' or, 'Today, it is Saturday' and sobota/niedziela is actually the subject (it doesn't really work in English...).
I don't think "to" strictly needs a noun in front of it... "to kot" = "this is a cat". I wonder though, can you say "To sobota" to mean "This is Saturday", and would it be too far of a leap for "Jutro to sobota" to mean "Tomorrow it will be saturday"? Sorry if I offer more questions than answers!
The "X to Y" construction works if both X and Y are noun phrases. "Jutro" is an adverb here. It could potentially be a noun (that's why it has a declension, as in "Do jutra" = "Until tomorrow"), but not here.
"Jutro to sobota" sounds as if you were trying to define the notion of "tomorrow" by saying that it is a Saturday :) But obviously that is not the definition of this word.