Generally, they are interchangeable. There is a nuance of difference between them. "Which" is used with a known selection of choices. - Which day of the month (1-31) are you going? "What" is not. - What day are you going? (no specific selection range) This distinction is not strictly used. Use whichever word you wish.

Your question should be : "When should I use 'What' and 'Which'? " If you are asking about the use of 'What..?' and 'Which..?' as question words, this is the difference: We use 'Which..?' when there is a limited range of options. For instance, if there are two drinking glasses on the table, you might say 'Which is your glass?' or 'Which glass is yours?'. By this, I mean 'the blue one or the green one?', for example. Or if you've bought tickets to see a band which is playing next week on Friday, Saturday and Sunday, I might say 'Which day are you going?'. But note that you could also use 'What' here. It's possible to say 'What day are you going?', too. In other cases, when there is a wide or unlimited choice, or we don't know what all the options are, we use 'What?'. For example 'What's your name?' 'What languages do you speak?' 'What's your job? and so on. This is because there are many possible answers. Note that you can't use 'Which' in most of these cases. It would be wrong to meet someone and say 'Which is your name?', because there's a virtually endless number of possible names which the person might have. If in doubt, use 'What..?' I hope that helps.

Hello Mr. Wright, I appreciate your correction. Thank you.

Your question is not phrased correctly. I recommend that you put some study time into how to ask questions in English. This will improve your English and make it more understandable. I think you are trying to ask this: =>When should I use "which" versus "what"?