gh-137481: consider actual day name length

Lib/calendar.py

@@ -370,7 +370,7 @@ def formatweekday(self, day, width):
         """
         Returns a formatted week day name.
         """
-        if width >= 9:
+        if width >= max(map(len, day_name)):
             names = day_name
         else:
             names = day_abbr

Lib/test/test_calendar.py

@@ -695,6 +695,36 @@ def test_locale_calendar_formatweekday(self):
             self.assertEqual(cal.formatweekday(0, 10), "  Monday  ")
         except locale.Error:
             raise unittest.SkipTest('cannot set the en_US locale')
+
+    # These locales have weekday names all shorter than English's longest 'Wednesday'
+    # They should not be abbreviated unnecessarily
+    @support.run_with_locale("LC_ALL", 
+            'Chinese', 'zh_CN.UTF-8', 
+            'French', 'fr_FR.UTF-8',
+            'Norwegian', 'nb_NO.UTF-8', 
+            'Malay', 'ms_MY.UTF8'
+    )
+    def test_locale_calendar_weekday_names(self):
+        max_length = max(map(len, (datetime.date(2001, 1, i+1).strftime('%A') for i in range(7))))
+
+        get_weekday_names = lambda width: calendar.TextCalendar().formatweekheader(width).split()
+
+        # Full weekday name, not an abbreviation, should be used if the width is sufficient
+        self.assertEqual(get_weekday_names(max_length), get_weekday_names(max_length + 10))
+
+        # Any width shorter than the longest necessary should produce abbreviations
+        self.assertNotEqual(get_weekday_names(max_length), get_weekday_names(max_length - 1))
+
+    # These locales have a weekday name longer than English's longest 'Wednesday'
+    # They should be properly abbreviated rather than truncated
+    @support.run_with_locale("LC_ALL", 
+            'Portuguese', 'pt_PT.UTF-8',
+            'German',  'de_DE.UTF-8',
+            'Russian', 'ru_RU.UTF-8',
+    )
+    def test_locale_calendar_long_weekday_names(self):
+        get_weekday_names = lambda width: calendar.TextCalendar().formatweekheader(width).split()
+        self.assertEqual(get_weekday_names(4), get_weekday_names(9))
 
     def test_locale_calendar_formatmonthname(self):
         try:

Misc/NEWS.d/next/Library/2025-08-06-16-54-22.gh-issue-137481.eSTkK0.rst

@@ -0,0 +1,2 @@
+Calendar uses the lengths of the locale's weekdays to decide if the width
+requires abbreviation.