PCS during Christmas season. Is it possible to fly to new duty station from leave city?

I did the same thing on a previous PCS 2 and oconus assignment. You have to talk to the transportation office. There is a charge to change the tickets to your new departure location. Plus, if the total of the new tickets cost more, you have to pay the difference as well. In addition, if you fail to make it to your Port call for any reason, you will be responsible for the cost of Transportation to your new Duty station.

The DTS folks might conduct a cost analysis to what is favorable to the government.

You should be talking to the transportation office that is booking your ticket because sometimes they can work stuff out.