Add the following code to your theme’s functions.php file. The code works from version 2.3.9 and upwards.

add_filter('mbhi_hours_entry', 'mbhi_add_day_suffix');
function mbhi_add_day_suffix($entry) {
	
   if($entry->type === 2 || $entry->type === 3) {	
      $entry->range = preg_replace_callback('/(\d+)/', function($matches) {
         $day = ($matches[0]);
         $last_digit = substr($day,-1);
			
         if($last_digit === "1")
            return $day . "st";
         if($last_digit === "2")
            return $day . "nd";
         if($last_digit === "3")
            return $day . "rd";

         return $day . "th";
      },$entry->range);	
   }
   return $entry;

}