Find the car you want, then buy it then and there. I don't buy into this whole time of day/month/year nonsense, I just find a car I want at a price I'm happy paying and go from there. Saving a couple of hundred quid is nothing over the lifetime of the car, and I'd rather get the one I want rather than one I find okay.
Of course, some people get really lucky and get both, never happened to me yet though!