FedEx may have promised delivery by Christmas. They shouldn’t have promised something they couldn’t deliver. But people can’t expect miracles when they order at the last minute, especially during the winter season when weather can cause major delays both in air and ground travel.
The cutoff date should be around December 15 or so instead of what? December 21?
The problem is that their promises depend on perfection. Any potential delays for weather or traffic tie-ups or the unforeseen aren't factored in.