"Sunday is a boring day."
Translation:Niedziela to nudny dzień.
That's just the Instrumental form of "dzień". Actually it's "dzień" itself that looks different than the other cases, all others start with dni-.
https://en.wiktionary.org/wiki/dzie%C5%84#Declension (you can see a form 'dnie' there, but it is quite literary, rather not used in contemporary language)