Interesting graphic. Buy a car in August on a Sunday, lol.
Typically, because of seasonality and model-year changeovers, the end of summer is the best time to buy a new car because dealers are trying to clear out their aging inventory to make room for the next model year. With such a large volume of vehicles crossing showroom floors, dealers can offer a great deal of price flexibility, making August the best time to get a great deal.
http://blog.truecar.com/2014/07/22/august-may-be-the-best-time-to-buy-a-new-car/