Strange behaviour why hook is not working

[Juergen]

Hello,

I have a hook which should copy the value of one field to another on special type of templates:

$pages->addHookBefore('saveReady', function($event)
{
    $page = $event->arguments(0);
    $page->setOutputFormatting(false);
        if(in_array($page->template->name, array('event_specialbusinesshours', 'single-special-business-hours'))) {
           $page->endeventdate = $page->starteventdate;
        }
});

The field value will be copied -> this works. The 2 fields are requiered. After pressing the save button I got the following message that the value is missing, but it is still there

To eliminate this error I could set the second field to not required, but I want an explanation why a copied value with "hook before saveready" leads to such an error-message. Does the validation take place before this hook. If yes, which hook method should be used instead?

Best regards