Asking what day of the week (e.g., 'What day is it? / Which weekday?').
明日は何曜日ですか？
What day of the week is tomorrow?
来週の何曜日が都合がいいですか？
Which day of next week is convenient for you?
このクラスは何曜日に開講しますか？
On which weekday is this class held?