gh-148663: Document that IllegalMonthError inherits from both ValueError and IndexError

Author: EoinTrial

Doc/library/calendar.rst

@@ -580,8 +580,11 @@ The :mod:`!calendar` module defines the following exceptions:
 
 .. exception:: IllegalMonthError(month)
 
-   A subclass of :exc:`ValueError`,
+   A subclass of both :exc:`ValueError` and :exc:`IndexError`,
    raised when the given month number is outside of the range 1-12 (inclusive).
+   The :exc:`IndexError` base class is preserved for backwards compatibility
+   with code that caught :exc:`IndexError` for bad month numbers prior to
+   Python 3.13.
 
    .. attribute:: month
 

Misc/NEWS.d/next/Documentation/2026-04-17-02-28-55.gh-issue-148663.MHIbRB.rst

@@ -0,0 +1,5 @@
+Document that :class:`calendar.IllegalMonthError` inherits from both
+:exc:`ValueError` and :exc:`IndexError`, preserving :exc:`IndexError` for
+backwards compatibility with code that caught it for bad month numbers prior
+to Python 3.13.
+###########################################################################