Should you advertise only on the quiet days?

Yes, you advertise the day that is empty, with time for people to book. It is not switched on only that morning if people set the table aside the day before and by then nobody is answering. Look in the book which day is booked and how far ahead. The ad goes out on Tuesday at two, when the shift has already started empty. The ad’s days and the hour it is switched on, beside the book.