add_action( 'pre_get_posts', 'wpse183601_filter_category_query' );
function wpse183601_filter_category_query( $query ) {
    // only modify front-end category archive pages
    if( is_category() && !is_admin() && $query->is_main_query() ) {



$today = current_time('Y-m-d'); 

      //query 1

      $args1 = array(      
           'meta_key' => 'Окончание', 
           'meta_type' => 'DATE',
           'orderby'=> 'meta_value',
           'order' => 'ASC', 
           'meta_query' => array(
                 'key' => 'Окончание',  
                 'compare' => '>=', 
                 'meta_type'  => 'DATE',
                 'value' => $today
             )
      );

      $args2 = array(      
           'meta_key' => 'Окончание',
           'meta_type' => 'DATE',
           'orderby'=> 'meta_value',
           'order' => 'ASC', 
           'meta_query' => array(
                 'key' => 'Окончание',
                 'compare' => '<', 
                 'meta_type'  => 'DATE',
                 'value' => $today
             )
      );

      global $wp_query;

      $query1 = new WP_Query($args1);
      $query2 = new WP_Query($args2);

      $wp_query = new WP_Query();

      $wp_query->posts = array_merge( $query1->posts, $query2->posts );

      $wp_query->post_count = $query1->post_count + $query2->post_count;




    }
}