Seasonality in Hiring?

It seems like there are good and bad months to be looking for a job. Anyone know when hiring is in season, and when it isn't? I'm particularly interested in how hiring seems to freeze up near the end of the year, and when it should resume again.
I once saw data from LinkedIn that showed January, May, September (in that order) were hiring/position peaks. I was seeing this when I had Premium subscription. Don't know whether it was based on industry/sector/job types that I was interested in/searching for or not. But I am sure it was geography-dependent.