The Donut Ride in Toronto is legendary. It's been going Saturdays and Sundays for more than thirty years, and is only prevented by heavy rain or snow. In summer they get more than a hundred riders, and in winter they still get dozens.
Here's footage from a ride in January. It was -8 degrees. That's cold when you're going 30+ km/h, so you need to dress properly for it. I myself have ridden in -10 before, but never for as long as the Donut Riders do (and, sadly, never as fast, either).
Basically, as long as the roads are dry, you can ride.
• Video Link
Donut Ride