'Mark every month in *bymonth*; *bymonth* can be an int or
sequence.  Default is ``range(1,13)``, i.e. every month.
*interval* is the interval between each iteration.  For
example, if ``interval=2``, mark every second occurance.'

def __init__(self, bymonth=None, bymonthday=1, interval=1, tz=None):

    if (bymonth is None):
        bymonth = range(1, 13)
    elif isinstance(bymonth, np.ndarray):
        bymonth = [x.item() for x in bymonth.astype(int)]
    rule = rrulewrapper(MONTHLY, bymonth=bymonth, bymonthday=bymonthday, interval=interval, **self.hms0d)
    RRuleLocator.__init__(self, rule, tz)
