Federal Express (NYSE: FDX) is gearing up for an unprecedented demand on its systems on Sept. 21, ostensibly from sales of Apple’s (NASDAQ: AAPL) impending iPhone 5, according to a published report citing an internal memo issued by the package carrier.

Last week Apple emailed invitations to the media for a Sept. 12 event at the Yerba Buena Center in San Francisco. The invitation read, “It’s almost here,” with a “12” casting a shadow of a “5,” referring, of course, to the iPhone 4’s successor, in all probability called the iPhone 5. Sept. 21 is the device’s expected release date.

In its note to employees, initially posted here, FedEx did not identify the “surge volume event between Sept. 21-24,” but it did postpone what appears to be training classes during the period. As noted in the report, FedEx generally is Apple’s carrier of choice for U.S. orders, although the vendor does deploy other services such as UPS in cases of high shipping volume.

The iPhone 5 is expected to be the biggest product launch in Apple’s history, which might explain why FedEx is going to extended lengths to prepare for the onslaught of packages likely to come its way. Sales estimates for the mobile device are off the charts, with the buildup months in the making both on Apple’s and customers’ part, as millions are said to have delayed purchasing smartphones waiting for the new version. Indeed, some predictions are for iPhone sales to not only reach 10 million units in the 10 days from its release to the end of the month but also easily eclipse the 4 million iPhone 4S units sold in its first three days on the market.

Meanwhile, other reports last week suggested that in the iPhone 5’s wake Apple will no longer offer the iPhone 3GS, although another report offered that the vendor will continue to sell the device in emerging markets.