Todd Defren's blog is always worth a read. Todd is one of the most experienced PR people in North America. Better still, he's gained the miles on the clock without losing his sense of humour. In this candid piece he describes Q4 mentality among his clients.
I would add a couple thoughts to Todd's. Firstly, it's not just clients who get a touch of the 'Q4's. It's colleagues, suppliers, partners, everyone. Secondly, it's not one dip. It's two. The second comes just after Christmas in week three of January. It's when everyone gets their credit card bills for the yuletide binge. This is not just a PR or media effect (although young-cultured, marketing organisations do feel the impact more). Many of the recessions in the last fifty years have been triggered by the onset of Winter in the Northern Hemisphere, because that's the location of the world's financial and commercial centres.
So just grin and bear it? No, anticipate it if you can. You can pay annual bonuses in November which gives a boost before Christmas and helps offset the inevitable tendency to overspend. You can also attend sales kick-offs in January and adjust working practices. Different people respond to different things. Sometimes, it's just enough to be aware of it and blog it out like Todd, whom I thank for his honesty and good humour.