diff -r ae76a1046bb9 Lib/calendar.py
--- a/Lib/calendar.py	Wed Mar 16 14:30:16 2016 +0100
+++ b/Lib/calendar.py	Sun Mar 27 12:34:25 2016 +0200
@@ -159,7 +159,10 @@
         date = datetime.date(year, month, 1)
         # Go back to the beginning of the week
         days = (date.weekday() - self.firstweekday) % 7
-        date -= datetime.timedelta(days=days)
+        try:
+            date -= datetime.timedelta(days=days)
+        except OverflowError:
+            pass
         oneday = datetime.timedelta(days=1)
         while True:
             yield date
diff -r ae76a1046bb9 Lib/test/test_calendar.py
--- a/Lib/test/test_calendar.py	Wed Mar 16 14:30:16 2016 +0100
+++ b/Lib/test/test_calendar.py	Sun Mar 27 12:34:25 2016 +0200
@@ -502,6 +502,10 @@
         # see #15421
         list(calendar.Calendar().itermonthdates(datetime.MAXYEAR, 12))
 
+    def test_itermonthdates_firstweek(self):
+        # ensure itermonthdates doesn't overflow before year 1
+        for i in range(1, 7):
+            list(calendar.Calendar(i).itermonthdates(1, 1))
 
 class MonthCalendarTestCase(unittest.TestCase):
     def setUp(self):