mem-category-order-plugin.php

<?php
/*
Plugin Name: MEM Category Order
Plugin URI: https://gist.github.com/
Description: Functionality plugin for https://wordpress.org/support/topic/ordering-post/.
Version: 0.1
Author: Manuel Schmalstieg
Author URI: https://ms-studio.net
*/

/**
 * This sample code is written in response to:
 * 
 * https://wordpress.org/support/topic/ordering-post/
 * 
 * "i would like to use your plugin but is there a way to order all the post of a specific category by the event start date?"
 * 
 * This example will order posts of the category with slug "events" by their start date.
 *
 * Caveat: this Category Archive will now only show articles that have a start date.
 *
 */
 
function mem_category_order( $query ) {

  if ( $query->is_category('events') ) {
  
  	$query->set( 'meta_key', '_mem_start_date');
  	$query->set( 'orderby', 'meta_value');
  
  }

}
add_filter( 'pre_get_posts', 'mem_category_order' );