The banks don't process any of my payments on the weekends, so bills and invoices don't matter if they arrive on the weekends.
Saturday and Sunday thus are not required.
As for legal documents, those are all based on the standard work week as well, so that argument falls as well.
As such there is no reason for mail to be delivered on Saturday or Sunday. At the company I work at we get mail on Saturday, we don't get to read it until Monday anyway.