Apple has had a few years to perfect the timing of all deliveries. They've been doing it for iPads and iPhones now. Even UPS knows how long it takes to get to each household from each warehouse, logistically. Of course, there are exceptions like weather. Deliveries now happens on the same day the item is available in store, generally speaking. I think in this case, there might not be any in-store stock, or very limited, but most of everyone who ordered in time will get it on the 24th, for all those watches that are sitting in the warehouses.
This is what the UPS warehouse looks like when the iPhone 5 was being prepped for delivery on the release date. UPS has been prepared for at least 2 years now for something like this.
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