I have sort of asked this before but I am needing some clarification.
If somebody were to ask "Sunday?" as a single word clarifying question is it still "w niedzielę?" or is just "niedziela?" correct. If it matters its more asking "do you mean on Sunday?" as if maybe they didn't hear the day you said.
"Niedziela" is Nominative. "W niedzielę" is Accusative. Why it's "niedzielę", not "niedzielii" in the Dative resp. Locative, I'm honestly no longer sure-:) ALL weekdays though follow the same pattern:
w niedzielę w poniedziałek we wtorek w środę w czwartek w piątek w sobotę
If you wanted however to indicate "Until Sunday", i.e. "See you (formal or familiar) Sunday!", the form would be "Do niedzielii!", same ending as in the Dative and Locative-:) "Niedziela" is feminine. The other weekdays, respectively:
Do poniedziałKU! Do wtorKU! Do środY! (also feminine, like "niedziela") Do czwartKU! Do piątKU! Do sobotY! (fem.)