If you're planning to head home for Thanksgiving this year, you'd better book your flights quickly. As Condé Nast Traveler points out, there's a small window when holiday airfare is at its cheapest, and once that window closes, prices start to soar.

Google analyzed price trends from this year and last year to help travelers figure out the best time to book their flights. While pricing varies depending on where you live and the exact dates you want to fly, early September is generally "the sweet spot for booking your Thanksgiving travel," Thijs van As, product manager for Google Flights, tells Condé Nast Traveler.

To illustrate this point, Google's Flight Price Trends tool includes a graph showing how flight prices fluctuate depending on when they're purchased. If you live in one of the major origin cities that Google included in the tool—Chicago, Los Angeles, San Francisco, New York, or Washington, D.C.—you can head to the website and see for yourself when you should snag a plane seat.

A separate Google tool for mobile devices, called Thanksgiving Prices, provides additional information on whether airfare prices are "low," "typical," or "high," and whether they're expected to go up.

Last year, Thanksgiving flights were cheapest when purchased around mid-September, according to Patrick Surry, chief data scientist for the price-tracker app Hopper. And the longer you wait, the more you'll have to pay.

"If you wait until early November, it will cost you about $2 per day you wait," Surry tells CNBC. "In the final two weeks leading up to Thanksgiving, prices begin spiking by $5 per day."

[h/t Condé Nast Traveler]