Let's just say that car guys can kind of be like women when it comes to selling cars, very emotional. The good old internet will tell you that the best time to buy a new car is late summer just before the new model years are realeased. There's some truth to that. The manufactures usually put bigger incentives on them (although we've never seen bigger incentives than right now) and the dealerships are usually willing to give bigger discounts to clear out the last year models. The problem is that they are just that, last year's models. Used car facilities such as Hertz have been selling that same model year for 10 months or so even cheaper than the heavily incentivised prices.
Really what you need to look for are trends in the economy, especially retail trends. Because of the economy, right now is as good of time to buy a vehicle as I've ever seen. Dealerships must turn over their inventory very quickly in order to be profitable. In tough economic times, a good dealer will make nothing on a car just to get it off the lot and replace it with a new vehicle that he hopefully can make some money on.
Depending on the type of vehicle, the seasons change the values of cars. For example, in the late fall and all of winter, 4x4 vehicles typically sell for more in locations that get snow. In the spring and summer, you will see prices of convertibles and sports cars increase.
The last thing to watch is fuel prices. Four months ago if you wanted to buy a Toyota Prius Hybrid, you would have paid anywhere from $6,000-8,000 more than what they are selling for today. If fuel prices go back up, so will the price for a Prius. Also when gas prices were very high, SUV's and truck prices fell through the floor.
So yes, there can be better times of the year to buy certain types of vehicles, but there is no standard format. It is always changing with the economy, fuel prices, and demand. There are alway fair deals to be made no matter what time of year, you just need to work with the right people!
Hope this helps!