AppleInsider obtained the internal calendar pictured below from a source.
An internal company document shared with AppleInsider purportedly shows time off available for AppleCare employees for the latter half of September. As can be seen in the calendar, available time off drops off significantly starting Sunday, Sept. 15, and continues through Saturday, Sept. 28. The image provided shows the calendar being accessed via virtual private network from an internal Apple employee domain. Potentially identifying portions of the picture have been cropped out.
It's already been reported that Apple has started training its employees on iOS 7 and the dates indicated above align with vacation blackout dates at Verizon and T-Mobile.
Current rumors peg the release of Apple's new iPhones on September 20th.