When is the best time to buy a new/used car from a dealership?
Today there isn't really a best time to buy. You have the internet on your side for best prices and comparisions, and more delaers are going with no haggle internet pricing. Years ago it was the end of the month due to salesman wanting to meet their quotas. Maybe that's still true, but work a deal that your happy with and go for it.
Probably November or December when the new models are out and they are looking to unload last year's model. If you don't mind buying the last year model, you can get a better deal on a new car.