Example

ex.php

/* before doing this check if $_GET['event_id']  is set e.g.
if ( ! isset($_GET['event_id']) {
    // return or do error stuff
}
*/


$event_id=sanitize_text_field($_GET['event_id']);
$event_id=wp_unslash($event_Id);
$date = get_post_meta($event_id, '_mdjm_event_date', true );

/* or combine it all */
$date = get_post_meta(
	wp_unslash(
		sanitize_text_field(
			$_GET['event_id']
		)
	),
	'_mdjm_event_date',
	true
);