"what" is the correct question word when you want information about something. "which" is the correct question word when you choose between different options - more specific information. e.g. What time is it? What will you wear to the party? [We imagine a unlimited range of possibilities] Which day will you come? Which of your dresses will you wear to the party? [There is a limited range of possibilities] In informal speech and writing, "what" is commonly and acceptably used as an alternative to "which". In formal speech and writing, doing this would be considered poor style.

These are often interchangeable but using "which" is more correct here.