add_action( 'pre_get_posts', 'order_conciertos_by_date' );
function  order_conciertos_by_date( $query ) {
	if ( is_admin() )
		return;

	if ( $query->get( 'post_type' ) != 'concierto' )
		return;

	$today = current_time( 'timestamp' );
	$today = date( 'Ymd', $today );

	$meta_query = array(
		array(
			'key' => 'show_fecha',
			'compare' => '>=',
			'value' => $today,
			'type' => 'numeric',
		)
	);

	$query->set( 'meta_query', $meta_query );

	$query->set( 'meta_key', 'show_fecha' );
	$query->set( 'orderby', 'meta_value_num' );
	$query->set( 'order', 'DESC' );
}