Where do I change the due date for event invoices and can I set a fixed date instead of a variable date?
There is a setting in Administer > CiviContribute > CiviContribute Component Settings that lets you change from the standard 10 days (you will need to tick the "enable tax and invoicing box to see it). But that doesn't get a fixed date. If you aren't using the invoices for something else then you could edit the "Contributions - Invoice" template in the "system workflow messages" tab in Administer > Communications > Message Templates.  Or if you do use the invoices for other things you could perhaps put some logic in the template to deal with the different cases. Hope that helps.