diff --git a/ChangeLog b/ChangeLog
index cc4893b356..d09528a453 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Jun 11 19:34:51 2011  Tadayoshi Funaba  <tadf@dotrb.org>
+
+	* ext/date/date_core.c (Date::(ABBR_)?(MONTH|DAY)NAMES): should be usascii.
+
 Sat Jun 11 19:24:33 2011  Tadayoshi Funaba  <tadf@dotrb.org>
 
 	* ext/date/date_core.c: rewrote doc.
diff --git a/ext/date/date_core.c b/ext/date/date_core.c
index 52402c7a20..9f6793e206 100644
--- a/ext/date/date_core.c
+++ b/ext/date/date_core.c
@@ -8114,7 +8114,7 @@ mk_ary_of_str(long len, const char *a[])
 	if (!a[i])
 	    e = Qnil;
 	else {
-	    e = rb_str_new2(a[i]);
+	    e = rb_usascii_str_new2(a[i]);
 	    rb_obj_freeze(e);
 	}
 	rb_ary_push(o, e);
diff --git a/test/date/test_switch_hitter.rb b/test/date/test_switch_hitter.rb
index 8f7accf793..d4603a23df 100644
--- a/test/date/test_switch_hitter.rb
+++ b/test/date/test_switch_hitter.rb
@@ -385,6 +385,19 @@ class TestSH < Test::Unit::TestCase
   end
 
   def test_enc
+    Date::MONTHNAMES.each do |s|
+      assert_equal(Encoding::US_ASCII, s.encoding) if s
+    end
+    Date::DAYNAMES.each do |s|
+      assert_equal(Encoding::US_ASCII, s.encoding) if s
+    end
+    Date::ABBR_MONTHNAMES.each do |s|
+      assert_equal(Encoding::US_ASCII, s.encoding) if s
+    end
+    Date::ABBR_DAYNAMES.each do |s|
+      assert_equal(Encoding::US_ASCII, s.encoding) if s
+    end
+
     h = Date._strptime('15:43+09:00'.force_encoding('euc-jp'), '%R%z')
     assert_equal(Encoding::EUC_JP, h[:zone].encoding)
     h = Date._strptime('15:43+09:00'.force_encoding('ascii-8bit'), '%R%z')