My order for a Space Black 256GB iPhone 14 Pro Max went through at 8:05 am and my confirmation email is timestamped at 8:06 am.
Do you remember if you saw the delivery date before checkout? You can always call or chat with an Xfinity Mobile rep to find out what date it will be delivered.
For anyone who hasn't received the email yet, as long as you saw your delivery date for 9/16 when you placed your order, you'll get it on 9/16. Xfinity Mobile's site was getting hammered and was showing out of stock for a couple of minutes (this happened last year with 13 Pro Max for me, Apple's own site had similar issues) and finally showed all Space Black Pro Max iPhones with delivery on 9/16.