Index: src/wp-includes/default-filters.php
===================================================================
--- src/wp-includes/default-filters.php	(revision 34995)
+++ src/wp-includes/default-filters.php	(working copy)
@@ -157,6 +157,8 @@
 
 add_filter( 'wp_sprintf', 'wp_sprintf_l', 10, 2 );
 
+add_filter( 'date_i18n', 'wp_maybe_decline_date' );
+
 // RSS filters
 add_filter( 'the_title_rss',      'strip_tags'                    );
 add_filter( 'the_title_rss',      'ent2ncr',                    8 );
Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 34995)
+++ src/wp-includes/functions.php	(working copy)
@@ -159,6 +159,47 @@
 }
 
 /**
+ * Determines if the date should be declined.
+ *
+ * If the locale specifies that month names require a genitive case in certain
+ * formats (like 'j F Y'), the month name will be replaced with a correct form.
+ *
+ * @since 4.4.0
+ *
+ * @param string $date Formatted date string.
+ * @return string The date, declined if locale specifies it.
+ */
+function wp_maybe_decline_date( $date ) {
+	global $wp_locale;
+
+	/* translators: If months in your language require a genitive case,
+	 * translate this to 'on'. Do not translate into your own language.
+	 */
+	if ( 'on' === _x( 'off', 'decline months names: on or off' ) ) {
+		// Match a format like 'j F Y' or 'j. F'
+		if ( @preg_match( '#^\d{1,2}\.? \w+#u', $date ) ) {
+			$months = $wp_locale->month;
+
+			foreach ( $months as $key => $month ) {
+				$months[ $key ] = '#' . $month . '#';
+			}
+
+			$date = preg_replace( $months, $wp_locale->month_genitive, $date );
+		}
+	}
+
+	// Used for locale-specific rules
+	$locale = get_locale();
+
+	if ( 'ca' === $locale ) {
+		// " de abril| de agost| de octubre..." -> " d'abril| d'agost| d'octubre..."
+		$date = preg_replace( '# de ([ao])#i', " d'\\1", $date );
+	}
+
+	return $date;
+}
+
+/**
  * Convert integer number to format based on the locale.
  *
  * @since 2.3.0
Index: src/wp-includes/locale.php
===================================================================
--- src/wp-includes/locale.php	(revision 34995)
+++ src/wp-includes/locale.php	(working copy)
@@ -128,19 +128,33 @@
 		$this->weekday_abbrev[__('Saturday')]  = /* translators: three-letter abbreviation of the weekday */ __('Sat');
 
 		// The Months
-		$this->month['01'] = /* translators: month name */ __('January');
-		$this->month['02'] = /* translators: month name */ __('February');
-		$this->month['03'] = /* translators: month name */ __('March');
-		$this->month['04'] = /* translators: month name */ __('April');
-		$this->month['05'] = /* translators: month name */ __('May');
-		$this->month['06'] = /* translators: month name */ __('June');
-		$this->month['07'] = /* translators: month name */ __('July');
-		$this->month['08'] = /* translators: month name */ __('August');
-		$this->month['09'] = /* translators: month name */ __('September');
-		$this->month['10'] = /* translators: month name */ __('October');
-		$this->month['11'] = /* translators: month name */ __('November');
-		$this->month['12'] = /* translators: month name */ __('December');
+		$this->month['01'] = /* translators: month name */ __( 'January' );
+		$this->month['02'] = /* translators: month name */ __( 'February' );
+		$this->month['03'] = /* translators: month name */ __( 'March' );
+		$this->month['04'] = /* translators: month name */ __( 'April' );
+		$this->month['05'] = /* translators: month name */ __( 'May' );
+		$this->month['06'] = /* translators: month name */ __( 'June' );
+		$this->month['07'] = /* translators: month name */ __( 'July' );
+		$this->month['08'] = /* translators: month name */ __( 'August' );
+		$this->month['09'] = /* translators: month name */ __( 'September' );
+		$this->month['10'] = /* translators: month name */ __( 'October' );
+		$this->month['11'] = /* translators: month name */ __( 'November' );
+		$this->month['12'] = /* translators: month name */ __( 'December' );
 
+		// The Months, genitive
+		$this->month_genitive['01'] = /* translators: month name, genitive */ _x( 'January', 'genitive' );
+		$this->month_genitive['02'] = /* translators: month name, genitive */ _x( 'February', 'genitive' );
+		$this->month_genitive['03'] = /* translators: month name, genitive */ _x( 'March', 'genitive' );
+		$this->month_genitive['04'] = /* translators: month name, genitive */ _x( 'April', 'genitive' );
+		$this->month_genitive['05'] = /* translators: month name, genitive */ _x( 'May', 'genitive' );
+		$this->month_genitive['06'] = /* translators: month name, genitive */ _x( 'June', 'genitive' );
+		$this->month_genitive['07'] = /* translators: month name, genitive */ _x( 'July', 'genitive' );
+		$this->month_genitive['08'] = /* translators: month name, genitive */ _x( 'August', 'genitive' );
+		$this->month_genitive['09'] = /* translators: month name, genitive */ _x( 'September', 'genitive' );
+		$this->month_genitive['10'] = /* translators: month name, genitive */ _x( 'October', 'genitive' );
+		$this->month_genitive['11'] = /* translators: month name, genitive */ _x( 'November', 'genitive' );
+		$this->month_genitive['12'] = /* translators: month name, genitive */ _x( 'December', 'genitive' );
+
 		// Abbreviations for each month.
 		$this->month_abbrev[ __( 'January' ) ]   = /* translators: three-letter abbreviation of the month */ _x( 'Jan', 'January abbreviation' );
 		$this->month_abbrev[ __( 'February' ) ]  = /* translators: three-letter abbreviation of the month */ _x( 'Feb', 'February abbreviation' );