python
diff --git a/Doc/library/calendar.rst b/Doc/library/calendar.rst
@@ -544,9 +544,14 @@ The :mod:`!calendar` module defines the following exceptions:
 
 .. exception:: IllegalMonthError(month)
 
-   A subclass of :exc:`ValueError`,
+   A subclass of :exc:`ValueError` and :exc:`IndexError`,
    raised when the given month number is outside of the range 1-12 (inclusive).
 
+   .. versionchanged:: 3.12
+      :exc:`IllegalMonthError` is now also a subclass of
+      :exc:`ValueError`. New code should avoid catching
+      :exc:`IndexError`.
+
    .. attribute:: month
 
       The invalid month number.

diff --git a/Lib/test/test_calendar.py b/Lib/test/test_calendar.py
@@ -457,12 +457,17 @@ def test_formatmonth(self):
             calendar.TextCalendar().formatmonth(0, 2),
             result_0_02_text
         )
+
     def test_formatmonth_with_invalid_month(self):
         with self.assertRaises(calendar.IllegalMonthError):
             calendar.TextCalendar().formatmonth(2017, 13)
         with self.assertRaises(calendar.IllegalMonthError):
             calendar.TextCalendar().formatmonth(2017, -1)
 
+    def test_illegal_month_error_bases(self):
+        self.assertIsSubclass(calendar.IllegalMonthError, ValueError)
+        self.assertIsSubclass(calendar.IllegalMonthError, IndexError)
+
     def test_formatmonthname_with_year(self):
         self.assertEqual(
             calendar.HTMLCalendar().formatmonthname(2004, 1, withyear=True),

diff --git a/Misc/NEWS.d/next/Documentation/2026-04-17-02-28-55.gh-issue-148663.MHIbRB.rst b/Misc/NEWS.d/next/Documentation/2026-04-17-02-28-55.gh-issue-148663.MHIbRB.rst
@@ -0,0 +1,2 @@
+Document that :class:`calendar.IllegalMonthError` is a subclass of both
+:exc:`ValueError` and :exc:`IndexError` since Python 3.12.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Document that :class:`calendar.IllegalMonthError` is a subclass of both
		:exc:`ValueError` and :exc:`IndexError` since Python 3.12.