"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.