A few weeks back someone pointed out to me that cycling in winter is difficult in Portland, Oregon because of the low temperatures.
So, I looked into this. Portland's average low in January is 1 C (33.7 F). Groningen's average low in January is -0.6 C (32.9 F). I haven't found figures for Assen, but it is just a few km south of Groningen so it'll be virtually identical.
Last year we had several days of -12 C ( 10 F ) weather. Cycling continued, and ice-skating became popular. So, as you'll see in this video and several other examples, people cycle to go ice-skating.
It was quite cold today, and the radio reported that in some areas it is already possible to skate on natural ice, so there will already be scenes as in the video.
OK, so sometimes the weather really is too extreme in some places. However, mostly "it's too cold in the winter here" is just another of those excuses people use not to cycle, and not to provide for cyclists. It isn't necessary for everyone to make all their journeys by bike, but there are few places where the weather is really too much of a challenge for cycling in all four seasons.
Here in the Netherlands the cycling rate in winter for utility purposes is about 95% of the rate in the summer. Cyclists are supported by the cycle paths and roads being kept clear of snow, and cycling is thus just as convenient and safe as at any other time of year. That's why there is significantly more cycling in Assen and Groningen in the middle of winter, despite the conditions, than in Portland in summer.