Toward the end of the year, manufacturers can offer dealers more incentives to boost their final sales figures of remaining inventory, Lapolla says. This allows dealers to pass along that savings to consumers, a move that often results in end-of-the-year sales, cash-rebate offers and financing discounts.

While these offers may sound like a final price, it's key to remember the dealer is still making a profit on the car, Lapolla says. "Consumers still have room to negotiate."

Another factor that may make salesmen eager to negotiate in December: end-of-year standings. Salesmen want to make their numbers for the year look as strong as possible to earn end-of-the-year bonuses. At some dealers, their jobs may even depend on it, Lapolla says.

Winter deals often apply to used cars as well, says Dan Harman, director of demand programs at Mojo Motors in New York. "The end of the year is traditionally a slow time for used-car purchases," he says. "As a result, vehicles have been sitting on the lot for a longer period of time. The longer a vehicle has been on the lot, the more likely a dealer is to lower or negotiate on the price."