Returns a list of workday names.

    Arguments
    ---------
    first_day : str, default None
        The first day of the five-day work week. If not given, 'Monday' is
        used.

    Returns
    -------
    list
        A list of workday names.

def workdays(first_day=None):
    """Returns a list of workday names.

    Arguments
    ---------
    first_day : str, default None
        The first day of the five-day work week. If not given, 'Monday' is
        used.

    Returns
    -------
    list
        A list of workday names.
    """
    if first_day is None:
        first_day = 'Monday'
    ix = _lower_weekdays().index(first_day.lower())
    return _double_weekdays()[ix:ix+5]