They are both correct, but used in different contexts. "Which" is used for choosing. When the set of choices is well-defined, use "which", as in "Which day is best for you, Friday or Saturday?" , or "Which day of the week do you prefer?" "What" is used for identifying.
Also to know is, which day or which date?
In fact these two words have different usages. The word 'day' refers to any particular day in the week. On the other hand the word 'date' refers to 'the number of the day in a particular month'.
Additionally, when to use which vs what?
2 Answers. "Which" is more formal when asking a question that requires a choice between a number of items. You can use "What" if you want, though. Generally speaking, you can replace the usage of "which" with "what" and be OK grammatically.
What comes first day or date?
The date format in American English When you prefer to write the date in American English, usually the month comes before the day, then followed by the year. If we use the same example as before: The 6th day of the month September, in the year 2019, then the date in American English should be written as: Sept 6.
What time or which time grammar?
Asking for time usually an open question, so using what is better. I'll add that "at which time" can also be used to introduce a non-restrictve relative clause: The museum will remain closed until 6:00am, at which time we will open the doors to visitors.