According to Case-Mate's site (the "offending" page is now down), there is a very specific ship date for the iPhone 5C:
Est. ship date 9/20/2013 8:00:00 AMAssuming Apple were to release the iPhone 5C on that date, it would indeed begin opening stores at 8:00 a.m., the early opening its retail stores use for device debuts.
Apple has scheduled a press event for Sept. 10 at 10 a.m. PDT. Based on history, it will introduce iOS 7, perhaps on Friday the 13th or the following Monday or Tuesday, and ship the new device(s) on Friday, Sept. 20.
However, it seems that U.K. shippers, while leaking similar information, are intimating a delay for the iPhone 5S. While the iPhone 5C is expected to arrive on Sept. 20, it's said that Apple's new flagship phone will not arrive until Sept. 27.
It's possible the second date -- a week later -- could be for a refresh run of devices, meaning that the 5C and 5S could ship to retail simultaneously on Sept. 20, but there have been rumors of the iPhone 5S slipping to Sept. 27, anyway, possibly due to issues with the rumored new champagne gold iPhone 5S color.
There's more evidence that Sept. 27 might be the debut date for the iPhone 5S. Mobile Syrup is reporting that a discontinuation list for Canadian carrier Telus shows the 32GB and 64GB versions of the iPhone 5 being discontinued on Sept. 28.
Nothing, of course, is real until Apple makes it so, and the fact that the 16GB model is not listed leads to the presumption that -- unlike some analysts have said -- the iPhone 5 may hang around even with the introduction of the iPhone 5C.