UserJalen is pretty much exactly right.
At Worldport they have a storage area that is over 4x4 footballs fields. They are likely holding the shipments in SDF to ensure Friday delivery and not sooner. If they put them into the system early they run a greater risk of breaking contract with Apple and phones being delivered early. Rumor is that is what cost FedEx the contract to begin with, so they will not screw that up.
As someone else pointed out, Friday may be our Christmas, but it is nothing like Christmas rush at Worldport. They will easily handle all of our packages and be delivered on time Friday. Even planes leaving ZZ in the morning will make it in time.