code: def mark_day(self, day, month=None, year=None):
          
          self._remark_date(day, month, year, highlight=True)
docs: Marks the specified month day with a visual marker
          (typically by making the number bold).
          If only day is specified and the calendar month and year
          are changed, the marked day remain marked.
          You can be more specific setting month and year parameters.