def truncate_week(datetime):
    '''
    Truncates a date to the first day of an ISO 8601 week, i.e. monday.

    :params datetime: an initialized datetime object
    :return: `datetime` with the original day set to monday
    :rtype: :py:mod:`datetime` datetime object
    '''
    datetime = truncate(datetime, 'day')
    return datetime - timedelta(days=datetime.isoweekday() - 1)