December 25 is one of the cheapest days to fly during the entire holiday season. Not one of the most expensive — one of the cheapest. Most travelers get this completely backwards. The expensive days are December 22, 23, and 24, when everyone scrambles to reach family before the main event. By December 25, those same travelers are already sitting around a Christmas tree or eating a big meal. The planes still fly. Most seats are still available. Airlines drop prices to fill them. Here is how to use that window before it closes. The December 25 Price Drop Is…