In my area in NJ, the Fall market starts to slow down and the winter gets even slower through the holidays. Come January, things explode. I sell in a resort area so people buy in January and February so that they are in for summer. So, it really depends on your locale. I find that in my areas the sellers are most desperate around October because they know if they don't sell, they may hold it till Spring. On the whole though, it's an affluent area and the sellers don't get desperate. We have what everyone wants, the Atlantic Ocean.
So, it is not so much what time of year sellers are desperate, but their individual circumstances that determine when.