$_SERVER[‘HTTP_HOST‘] only exist when visit calendar via web page, so if you are trying to run cron jobs for event reminders, you will be using PHP CLI. If you getting the following warning

Warning after cron job execution “Notice: Undefined index: HTTP_HOST in pec.php…”

you will need to set the HTTP_HOST manually to point to your domain in pec.php

if ( defined( 'WP_CLI' ) && WP_CLI ) {
    $_SERVER['HTTP_HOST'] = 'yourdomain.com';
}