For popular cars that sell well you only have so-much leverage. It's a lot easier to get deals on cars that are tougher to sell, the dealer doesn't have a lot of incentive to deeply discount a popular model that he can sell for a higher price inside a week. That being said, right before Christmas is a decent time to buy as most people are shopping for anything except cars. It's a plus if you're in the north and the weather is bad, most people don't go car shopping if it's snowing or really cold. But there's no way to be sure. You're just as likely to walk into a dealer that has made all its sales quotas and doesn't need to deal.
Instead of trusting to luck, do your homework. The more prepared you are, the better your bargaining position. Know the prices and get some online quotes so you have a current frame of reference. The key to getting a good deal is knowing exactly what is really a good deal and what only seems like a good deal. That comes from knowledge, not timing.