$args = array(
  'post_type'      => 'tribe_events',
  'publish_status' => 'published',
);

function xxx_order_events( $query ){ 
  $query->set( 'orderby', 'meta_value' ); 
  $query->set( 'meta_key', '_EventStartDate' ); 
  $query->set( 'order', 'ASC' ); 
  return $query; 
} 
add_filter( 'pre_get_posts', 'xxx_order_events', 99 );

$var = new WP_Query($args);