If you're wondering when to book Christmas flights, the honest answer is that the date you fly matters more than the day you buy. December is a fixed-demand stretch, the same way Thanksgiving is. A specific set of days gets crowded, those days price accordingly, and holding out for a cheaper week doesn't help when the week itself is the problem. So the useful question isn't "what's the magic booking day." It's which dates inside the holiday window you're willing to fly.
When to book Christmas flights, and the dates that cost the most
Two clusters do the damage. The getaway days, roughly December 19 through 23, when everyone leaves at once, and the return days, January 2 through 4, when everyone comes back before work and school start again. Those are the peak of the peak, and the fares know it. If your trip has to land on a Friday-before out and a Sunday-after back, you're buying the most expensive version of the trip, and no amount of refreshing changes that.
The quiet discount is the holiday itself. Flying on Christmas Day, or on New Year's Day, usually comes in well under the days on either side, because far fewer people want to spend the morning on a plane. A departure on the 24th or 25th and a return mid-week in early January often beats the crush by a wide margin. It's the same logic behind the cheapest days to fly the rest of the year: the days nobody wants stay affordable.
Don't wait for a drop that isn't coming
The instinct to hold out is strong, and in December it's almost always wrong. The old last-minute fire sale barely exists on ordinary routes anymore, and over the holidays it essentially never happens. Planes are full. There's no reason to discount a seat someone else will happily pay for. We said the same thing about Thanksgiving flights, and the Christmas version is even more lopsided because the busy window is longer.
None of that means panic-buy the first fare you see. It means the downside of waiting is real and the upside is thin, so the move is the same one we keep landing on in how far ahead to book: decide the dates first, then watch them. Point Flight Watcher at your December dates now and you'll get the three best fares each morning, instead of opening six tabs in mid-December and hoping for a sale the airline stopped running years ago.