Planning a trip this Thanksgiving? You might want to start checking air fares in the next few days. Travel websites and apps that track the optimum time to book travel report that September will be the best time to get a Thanksgiving deal on air fares. 

Why? Deciding when to buy plane tickets is always a bit tricky. Shell out your money too soon and you might miss out on good deals as airlines compete with each other to fill seats. But wait too late and prices will climb as travelers compete for the last few empty seats. 

Apps like Hopper and services like Google Flights try to solve this conundrum by using algorithms to predict the optimum time to buy tickets for some of the nation's most popular routes, using data from last year as well as this year to make that determination. Patrick Surry, chief data scientist at the company that makes Hopper, told CNBC that last year, the perfect time to buy a Thanksgiving airplane ticket was about September 15. 

Similarly, Google Flights' new price trends tool attempts to predict how prices will change in the time before you book a flight, and thus predict when you should buy the ticket. Its algorithm suggests that early September is the best time to book a Thanksgiving flight, with prices climbing steadily through the month.

While it's sometimes possible to get a good deal on a flight by waiting until the last few days, don't try this with holiday travel, experts warn. You can expect to pay top dollar for your Thanksgiving travel if you book it after October 31, with prices rising fastest the closer you get to the holiday.

You probably already know that you can save some money if you're flexible about your travel dates. Google now offers something called Thanksgiving Prices to help you figure out just how much you can save by adjusting your travel dates. Please note that it's only available on mobile devices. Using this tool, you will likely see that you could save some money by flying out on Thanksgiving morning rather than the Wednesday before the holiday. That seems like a no-brainer, although given the possibility of flight delays, it also could mean missing Thanksgiving dinner.

But apparently, the real budget killer is planning a return trip during the weekend after Thanksgiving. If you can arrange to fly home on the Monday or Tuesday after the holiday, you may see significant savings.

So what are you waiting for? It may still be a hot summer day outside, but Thanksgiving will be here before you know it. Start shopping for flights now and you may be able to save some cash.