I presently possess the following code for blocking posts for any home page in Wordpress. However, I wish to order with a custom taxonomy EventStartDate rather than their default ones (i.e. date, title, etc.).

$cat_posts = new WP_Query(
   "showposts=" . $instance["num"] . 
   "&cat=" . $instance["cat"] .
   "&orderby=" . $sort_by .
   "&order=" . $sort_order

Any help could be much appreciated!

Custom Taxonomy Sort wordpress plugin should have the desired effect.