Aug 22
Yes, there will be a scrimmage on Aug 22nd.
Everyone who has season tickets will also get a ticket for the scrimmage. Athletics is planning to use this event as a dry run for all stadium operations. They will scan tickets, run concessions, etc. (I think that if you have also purchased parking you'll get a pass to your lot as well--but don't remember for sure.)
They want to use this as an opportunity to get any remaining kinks out of the system prior to the grand opening. If there are any remaining issues they'll have some time to address them before the first game.
It will be the first opportunity most people will have to get into the stadium. Fans will be able to find and sit in their seats, take their time and look around the new digs, etc.