It has always been recommended to submit ASAP after it opens on Sept 15th. Some programs start responding very early (end of september). With that said, I know applicants who still received plenty of interviews with a delay of a week or 2 in their applications. If you are able to get it all in on the 15th I would urge you to do so.
As early as possible. Despite an animate corpse being able to match radiology right now the top programs, both by reputation and location, are still somewhat difficult to match.
agree with above.. the application is open now, there's no reason not to have it complete by september.. some programs start offering interviews that same week, so if they fill their spots and you didn't submit yet, that's kinda your fault