I don't know that there is a best month, but definitely towards the end of the year, you can get better deals on current year models, and the first months of a new year, great deals can be made on left over vehicles from last year. A lot of dealerships take out loans in order to buy new inventory, and therefore, must pay interest every month the vehicle sits on their lot. And every month they have to pay interest on a vehicle, that eats into the amount of potential profit they could be making on the vehicle. The last thing a dealer wants is to owe the bank more for a vehicle (principal + interest) than they would be able to sell it for. There isn't really any way for us buyers to find out when that point is on any particular vehicle, but know that the longer a vehicle sits on a dealer's lot, the closer the dealer is getting to that point, and the more motivated they are to get it off.
Of course, waiting until the end of the year/first of a new year to purchase the current/previous year model means that the selection is going to be slim and options are going to be few. It's just like shopping the after Christmas sales. If you don't mind picking through the leftovers in order to score a great deal, then this tactic could be for you. If you are the type who wants the hottest new thing with the best options, you are going to have to pony up and pay the higher prices for it.