code: def _check_holidays(holidays):
      """Checks that holiday files are in the correct format.
  
      Args:
        holidays: a Pandas `DataFrame` containing the dates of holidays.
  
      Raises:
        ValueError: if the holidays column names are improperly formatted.
      """
      all_column_names = _HOLIDAY_FILE_FIELDS.issubset(holidays.columns)
      if not all_column_names:
          raise ValueError(
              "Holidays column names must contain: {0}.".format(
                  _HOLIDAY_FILE_FIELDS
              )
          )
  
label_desc: Use image node_id 4 for calling a global function with example usage: _check_holidays(holidays) without return types