def METHOD_NAME(data_dir):
    """
    Load all months and return a directory mapping year-month values
    to month objects.
    """
    months = {}
    logging.debug(f'Starting to load files in dir "{data_dir}"')
    for path, year_number, month_number in get_journal_files(data_dir):
        if month := _load_month_from_disk(path, year_number, month_number):
            months[format_year_and_month(year_number, month_number)] = month
    logging.debug(f'Finished loading files in dir "{data_dir}"')
    return months