def weekdays(first_day=None):
    """Returns a list of weekday names.

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

    Returns
    -------
    list
        A list of weekday names.
    """
    if first_day is None:
        first_day = 'Monday'
    ix = _lower_weekdays().index(first_day.lower())
    return _double_weekdays()[ix:ix+7]