Wow, did you have any indication in any other correspondences that you were up for an interview? (GCMS notes? I think you didn't have access to GCKey account, right?) I wonder why some people were notified that they were in queue and not given a date, but with you they just gave you the interview date without notifying you first that you were in queue?
At least you will know in October probably what decision they will come to, so it is almost the end of the journey for you! Hope all goes well for you next month!