functions.php

<?php
add_action( 'pre_get_posts', 'tribe_post_date_ordering', 51 );
function tribe_post_date_ordering( $query ) {
	if ( $query->tribe_is_multi_posttype) {
		remove_filter( 'posts_fields', array( 'TribeEventsQuery', 'multi_type_posts_fields' ), 10, 2 );
		$query->set( 'order', 'DESC' );
	}
}