For me it tends to start just after Christmas (before NYE which I HATE) and lasts until probably mid April. Everything feels so bleak and empty and pointless and cold. I've already got the feelings of dread starting now even though I actually really love Autumn. But my BFF suffers in Autumn. Late October and November is worse for her but the shortest day is a turning point. She actually likes Jan and sees it as a fresh start. I tend to get the fresh start thing in Sep, probably a hangover from school days. I really REALLY struggle to think of a single good thing about January.
So really I'm just curious to see which is more common.