If history is any guide, there will be "floating" demos; i.e., in the hands of employees, not tethered to the table, on the day they go on sale. A full table of demos will be available next day.
I'm assuming Apple will do the typical Friday 6pm launch, close all stores a 4pm deal. However, if Apple puts them on sale at regular store opening, then maybe employees will have set up a demo table the night before.
What is guaranteed is that Apple will not have demo units available until it actually is available for sale.