diff --git a/ChangeLog b/ChangeLog
index 4fbb871235..17075925fa 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2018-03-16  Rafal Luzynski  <digitalfreak@lingonborough.com>
+
+	[BZ #22963]
+	* localedata/locales/cs_CZ (mon): Rename to...
+	(alt_mon): This.
+	(mon): Import from CLDR (genitive case).
+
 2018-03-16  Rafal Luzynski  <digitalfreak@lingonborough.com>
 
 	[BZ #22937]
diff --git a/localedata/locales/cs_CZ b/localedata/locales/cs_CZ
index f80d3e1b37..8f4c69e493 100644
--- a/localedata/locales/cs_CZ
+++ b/localedata/locales/cs_CZ
@@ -272,7 +272,7 @@ day		"Ned<U011B>le";/
 		"P<U00E1>tek";/
 		"Sobota"
 
-mon		"leden";/
+alt_mon		"leden";/
 		"<U00FA>nor";/
 		"b<U0159>ezen";/
 		"duben";/
@@ -334,6 +334,19 @@ abmon		"led";/
 %		"Nov";/
 %		"Dec"
 
+mon		"ledna";/
+		"<U00FA>nora";/
+		"b<U0159>ezna";/
+		"dubna";/
+		"kv<U011B>tna";/
+		"<U010D>ervna";/
+		"<U010D>ervence";/
+		"srpna";/
+		"z<U00E1><U0159><U00ED>";/
+		"<U0159><U00ED>jna";/
+		"listopadu";/
+		"prosince"
+
 week		7;19971130;4
 first_weekday	2