Allow me to rephrase it:
First, just in case a mistake is made, make a copy of wp-includes/classes.php and name it untouched_classes.php. Now open wp-includes/classes.php in a text editor (notepad) and FIND
if (mysql2date('U', $this->posts[0]->post_date_gmt) > mysql2date('U', $now)) { //it's future dated
$this->is_preview = true;
AFTER that, using "copy and paste", add the following:
/* DIRTY FUTURE-POSTS HACK FOR EC3 */
if (function_exists('ec3_get_calendar')) {
global $ec3;
$ec3_post_0_is_event = intval($wpdb->get_var(
"SELECT COUNT(0) FROM $wpdb->post2cat WHERE post_id="
.$this->posts[0]->ID." AND category_id=$ec3->event_category"));
if ($ec3_post_0_is_event) {
$this->is_preview = false;
}
}
if ($this->is_preview)
/* DIRTY FUTURE-POSTS HACK FOR EC3 */
Save and upload the file to your wordpress folder.
Hope that helps!