"Sunday" as an answer to the "when" question would be "w niedzielę" (accusative case).
Sorry for the slight necro. But may I ask why it would be "w niedziełę" and accusative? (I know that you're correct, I just don't understand why) I thought when followed by the preposition w(in) something must become locative? even if it's a response to the question "when" I.e, "Kiedy będziesz napić? " - "W Wrocławiu"