Because if somebody trys to pre-order tomorrow, and fails, then we'll know the last day to pre-order is tomorrow. If they succeed...it isn't tomorrow is it?
I would assume that all the shipments for the 19th would shipped tomorrow (Monday). If you can still order on Monday and have delivery by the 19th, then the cut off will either be Wednesday or Thursday (if you want over night shipping, if that option is offered).
I think apple should have made this clear. I ordered on the 10th I believe, so I'm set. Everyone should order today if Friday delivery is a must.