I'm riding all year, although longer trips get significantly harder below zero degrees Celsius...
Every winter (over here in the NL) only has a couple of days so cold and sh**ty that I go to work on a bicycle, or even take a day off...
The most important is to get the right clothing : an insulated motorcycle jacket, motorcycle gloves, and one or two of those windproof biker scarves... This dedicated stuff is really different from just putting on extra layers of everyday clothing, my first motorcycle jacket was a true eye opener. It not only keeps you warm really well, but it will also give some freedom of movement that a stack of regular clothes won't...
And take your time when there could be patches of ice everywhere.. Don't just go fast because everybody else does at a certain road. Most people are just ignorant and lucky, and the ones that aren't lucky will show up in the newspaper next day...