Buying plane tickets is almost like a game. If you book too soon, you could miss a deal later on. But if you wait too long, you might end up paying too much. It's especially true at this time of year, when a lot of us are making travel plans for Thanksgiving and Christmas.

So, when's the ideal time to pull the trigger on airfare?

Travel booking site Kayak says it has the answer: 21 days before your departure date.

Kayak looked at millions of flights booked by its users. Over a six-month period, the average fare for a domestic flight was $370. But three weeks before departure, the fare dropped to $342, its lowest level over the test period.

You can roll the dice and wait a little longer for a better deal, but it might cost you. Kayak says airfares really take off, by as much as 30 percent, in the week before your departure date.