production started on sept 7, depending on where the first one was sold any where from 2-3 days after that to 2 weeks I would imagine, but Jay Leno had his sooner than that.
Sept 7 was the "official" start of production, but almost a thousand 05 Mustangs had already been built before Sept 7.
Sept 27 was the official Mustang launch celebration where the media was invited to the assembly plant to see '05 Mustangs rolling off the line. However several thousand more '05s were built between Sept 7 and Sept 27.
Althoug the '05 Stang was officially launched on Sept 27, NONE shipped to dealers until almost 2 weeks later as they were ALL being held for final quality checks. If you search you can find lots of discussions about this here on Stangnet and on Brads site complete with photos of thousands of '05 Stangs sitting in the holding lots.
Finally, you can't instantly move thousands of Mustangs to 5,000 Ford dealers, so some dealers got '05 Stangs in early October while Redfire said, some dealers did not get them until November.