I suppose it depends on when you call. If it's before the cancellation time on Saturday, they would/could/should cancel your Saturday stay and re-book a 2 night stay for Sun-Tues. If it's after the cutoff, they might still charge you a no-show for Saturday and a 2 day stay afterwards. In that case, they'd probably offer, or you should suggest, ignoring your attempt to cancel and just keep it as a 3 night stay with a very, very late check-in.