diff --git a/src/wp-includes/widgets/class-wp-widget-calendar.php b/src/wp-includes/widgets/class-wp-widget-calendar.php
index 8fd7aebed0..4329bcf18a 100644
--- a/src/wp-includes/widgets/class-wp-widget-calendar.php
+++ b/src/wp-includes/widgets/class-wp-widget-calendar.php
@@ -7,6 +7,16 @@
  * @subpackage Widgets
  */
 class WP_Widget_Calendar extends WP_Widget {
+	/**
+	 * Ensure that the ID attribute only appears in the markup once
+	 *
+	 * @since 4.4.0
+	 *
+	 * @static
+	 * @access private
+	 * @var int
+	 */
+	private static $instance = 0;
 
 	public function __construct() {
 		$widget_ops = array('classname' => 'widget_calendar', 'description' => __( 'A calendar of your site’s Posts.') );
@@ -25,10 +35,16 @@ class WP_Widget_Calendar extends WP_Widget {
 		if ( $title ) {
 			echo $args['before_title'] . $title . $args['after_title'];
 		}
-		echo '<div id="calendar_wrap">';
+		if ( 0 === self::$instance ) {
+			echo '<div id="calendar_wrap" class="calendar_wrap">';
+		} else {
+			echo '<div class="calendar_wrap">';
+		}
 		get_calendar();
 		echo '</div>';
 		echo $args['after_widget'];
+
+		self::$instance++;
 	}
 
 	/**