Personally, never. Unless it's a brand new Ferrari/Porsche or whatever, I'll never buy a brand new car. For a run of the mill every day car, 2nd hand, even just a year old and you'll have a car that's still in warranty, still as good as brand new so reliable etc, but you'll save a good 25% off the purchase price.
If you are set on buying a brand new car though, I guess good times of year to go are just before the new registration plates come out, as there's stock to be shifted. I don't really think there's a lot be gained at any other times of the year, most places run fairly close to their profit line anyway, and they're not going to let you negotiate them into making a loss on a new car!