Apple typically sends out invitations 2 weeks prior to the event, so at this point its easy to cross off dates, i.e., anything before October 20th is probably not the date.
I have a sneaky suspicion we'll see a late Oct event with shipping dates closer to the end of the year. I have 0 data to back it up, but based on Apple's lax attitude toward the Mac, I wouldn't expect it to be shipping the day after the event.