code: def _pressed(self, evt):
          
          x, y, widget = evt.x, evt.y, evt.widget
          item = widget.identify_row(y)
          column = widget.identify_column(x)
  
          if not column or not (item in self._items):
              # clicked in the weekdays row or just outside the columns
              return
  
          item_values = widget.item(item)['values']
          if not len(item_values):  # row is empty for this month
              return
  
          text = item_values[int(column[1]) - 1]
          if not text:  # date is empty
              return
  
          bbox = widget.bbox(item, column)
          if not bbox:  # calendar not visible yet
              return
  
          # update and then show selection
          text = '%02d' % text
          self._selection = (text, item, column)
          self._show_selection(text, bbox)
  
          if self.callback is not None:
              self.callback()
docs: Clicked somewhere in the calendar.