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It's July. Time to start thinking about this Christmas purchase

  • The optimal window for booking affordable holiday flights is right now until mid-September, according to Cheapflights.com.
  • Take these travel suggestions with a grain of salt, a travel expert says.

The Christmas holiday season may seem far away while you soak up the sun on summer vacation. But winter is coming — and the time to snag holiday flight deals is now.

The window for booking affordable holiday flights is early July through mid-September, according to Cheapflights.com, a flight search and travel deals web site. Optimal times for the most popular holiday destinations vary within that timeframe (see chart below).

Even if you'll be flying to a small city rather than of one of the big cities listed, you should still buy your tickets early, said site editor Melisse Hinkle.

Airplane landing
Toby Melville | Reuters

"By and large, the holiday season is the most expensive time to fly to destinations large and small, particularly to ones in the U.S," she said. "This is prime travel time no matter where you are going, and so the need to plan ahead holds true for smaller markets as well."

Take calculated travel guides like this one with a grain of salt, though, said George Hobica, president of fare-tracking site Airfarewatchdog.com. In a June post, he warned consumers, "There is no magic formula."

Instead of blindly buying your ticket during a suggested week, Hobica said, sign up for fare alerts and pay attention to sales. Be prepared to buy when a good price pops up.

"Start tracking prices as they go up and down — and they will certainly go up and down," he said.

But don't wait too long. Although you may still see sale alerts even a few weeks out from the winter holidays, last-minute fares can be pricey. Plus, you run the risk that those late sales may leave you with a middle seat, a long layover or route through a winter-storm-prone city.

"The risk of waiting too long is fairly straightforward: you're likely to pay more," said Hinkle at Cheapflights.com. "That's not to say there won't be one-off fare sales across travel newsletters and social media channels, but this is a time of high travel demand so there isn't much incentive for airlines to offer deals for holiday season travel."

— By Abigail Summerville, special to CNBC.com