ENH: Don't infer WOM-5MON if we don't support it (pandas-dev#9425)

Author: austinc

pandas/tseries/frequencies.py

@@ -926,6 +926,7 @@ def _get_wom_rule(self):
             return None
 
         week_of_months = unique((self.index.day - 1) // 7)
+        week_of_months = week_of_months[week_of_months < 4]
         if len(week_of_months) > 1:
             return None
 

pandas/tseries/tests/test_frequencies.py

@@ -210,6 +210,15 @@ def test_week_of_month(self):
         for day in days:
             for i in range(1, 5):
                 self._check_generated_range('1/1/2000', 'WOM-%d%s' % (i, day))
+            self._check_generated_range('1/1/2000', 'WOM-%d%s' % (i, day))
+
+    def test_fifth_week_of_month(self):
+        func = lambda: date_range('2014-01-01', freq='WOM-5MON')
+        self.assertRaises(ValueError, func)
+
+    def test_fifth_week_of_month_infer(self):
+        index = DatetimeIndex(["2014-03-31", "2014-06-30", "2015-03-30"])
+        assert frequencies.infer_freq(index) is None
 
     def test_week_of_month_fake(self):
         #All of these dates are on same day of week and are 4 or 5 weeks apart