At some point, typically 6 weeks or so in advance, the release date (or an approximate timeframe) will be given so that the Store can begin taking preorders.

By the way... There's no need to pay for expedited shipping. The preorders are always shipped out to be delivered the say of release. The only way to guaranty delivery on a certain day is for orders to go FedEx Air, so all orders are upgraded for free. This is how it's been done the last three releases anyway.

Note that OS releases often happen on a Saturday. Consider this when indicating a shipping address, i.e. don't ship to work if you're not there on Saturday. If the release is on a Saturday, shipments usually arrive to their destination before 10 AM. Believe it or not, previous recent OS releases have been a burden on FedEx nationwide, so that 10 AM delivery deadline could be a goal rather than a definite target. OTOH, sometimes a sprinkling of lucky orders arrive a day sooner, because a staged shipment accidentally gets processed too soon or something. The fraction of orders that arrive early or late is usually minuscule, and execution generally gets better each successive release. Fortunately, the entire operations and logistics wheel isn't reinvented each OS launch.

I can't speak to how it's happening now or when this go-round, and anything I've said about how it worked in the past one could glean from public sources. Just have to get that out of the way.