The date presumably is April 18 based on past history and ND's academic calendar. There often isn't an official announcement of the date.
But if an off-campus location were set, presumably there'd be a formal announcement, particularly if the game will be at Soldier Field. I'd be very surprised if ND quietly moved the game to Chicago without a push for ticket sales, which they'd presumably need to recoup the rental fee. The longer we go without an announcement of location, the more likely it is that the game will be somewhere like St. Joe's HS, or simply at Loftus.