You can t. It s not possible. The USPS is run by the government and won t do it for you. You can plead to the organizers to let you in, but other than that? Not going to happen.
You can really backdate any piece of mail, but you can obscure the date by placing a dozen or so stamps where the date cancelling stamp goes You get a bunch of low value stamps and cover the entire 4 inches from top right right side with them. Good Luck!
Use Einstein Express (an old Saturday Night Live fake commercial), an express courier service that handles late-arriving packages by literally sending them back in time to the desired arrival date. The slogan: When it absolutely, positively has to be there the day before yesterday.
