On the iPhone page at Apple's website, you can see that Apple posts that interested buyers can purchase the iPhone 5s from Apple Online Store starting 12:01 am PST. The rumored reason as to why Apple may be doing this is because of supply problems. Since the iPhone 5s includes the fingerprint sensing technology, Apple's partners who manufacture the 5s are having issues putting the device together at a fast pace.
Due to this issue, the company has decided to make the device available on a first-come first-serve basis. If you have an Apple store near you, then you may already see lines forming in front of the store. Since there are supply issues, if you can get your hands on the device on September 20, then you're lucky, but if you don't get the device on launch day, then you will have to wait until a new shipment comes in.
Apple Stores will open 8:00 a.m. local time to begin selling the new iPhone 5s. If you're looking to get the 5s, better get in line now.