because they are a huge seller of phones. if you are Verizon or HTC and want to sell product, you want to get your product into Costco. They would know because it is in their systems.
that is asking why would best buy know or why would any retailer know. if you start calling other retailers you might find confirm the date. however, they are usually sworn not to release that information. what we need is someone on the inside to leak a photo of the date on the computer screen, that will squash all doubts.
look, if the phone doesn't come out next week i give everyone the right to call me a big douce, until then, well please just wait, don't call me one.