Here are some important shipping facts...
Standard shipping for both UPS and FedEx from China to US is 5 days. They both have quicker services that has been seen in iPhone shipments - but it is not overly common. Customs clearance is automatic and does not cause a delay. Once the package hits the hub in Memphis or Louisville, it takes one day to get to any domestic address in the 48. It can be same day if plane arrives in the early wee hours of the morning. With iPhones, one carrier is primary and the other carrier handles overflow. (Last iPhone, UPS was primary and FedEx handled a minority of deliveries).
Tracking numbers are often issued 2 days prior to the shipment. Sometimes Apple gives it to you then and sometimes they wait until package is already on its way. Example: my iPhone 6 was delivered Oct 24 and I received tracking from Apple on Oct 15. I was in the 2nd wave of orders. I believe most of the 1st wave of orders received tracking numbers only a couple days before delivery.
Once the package is in transit, you can have the carrier hold for pickup at your local hub rather than deliver it. I do that because I can pick it up at 8 am instead of have it delivered around 4 or 5. Yes, a few hours matters.
It is my educated guess that a lot of initial shipments will start leaving tomorrow (Thursday) through Monday for a 24th delivery. The biggest challenge for the carriers is not the actual delivery, but getting the large quantity of containers on planes to the U.S. Once they are in the U.S., it's just business as usual for the carriers.