The correct phrase depends on the context.
1. **’What day’** is used when you are asking for specific information about a day, such as the day of the week or the date. For example, ‘What day is the meeting?’ or ‘What day is it today?’
2. **’Which day’** is used when you are choosing between specific days. For example, ‘Which day works best for you, Monday or Tuesday?’
In summary, use ‘what day’ when asking for general information about a day and ‘which day’ when selecting from specific options.