When do pre-order movies become available

You will get an e-mail notification.

When your pre-ordered item releases, your order completes and the payment method that you have on file in the iTunes Store is billed. You'll receive an email letting you know that your pre-order is ready to download. Click or tap the link in the email to download your item.

The page that you posted from has instructions for how to see if a release date has been set for it - if nothing shows the film studio/rights-holder either hasn't set one yet, or have not added it to the film in your country's store