Source

File: wp-includes/widgets/class-wp-widget-calendar.php

public function __construct() {
		$widget_ops = array(
			'classname'                   => 'widget_calendar',
			'description'                 => __( 'A calendar of your site’s posts.' ),
			'customize_selective_refresh' => true,
		);
		parent::__construct( 'calendar', __( 'Calendar' ), $widget_ops );
	}