何日 is a literary or formal interrogative pronoun meaning 'what day?' or 'which day?' (written form of 哪一天). It appears mainly in classical-style writing, poetry, formal letters, or historical narratives. Unlike the everyday 哪一天, 何日 carries a slightly archaic or elevated register, and it often implies a rhetorical or future-oriented question (e.g., 'when will we meet again?'). It is used in the same position as 哪一天 within a sentence — typically before the verb or at the start of a question.