It was freezing on January 25, 2018. It recorded a low of negative 4 degrees celsius (-4℃). It shows -3℃ in Chiyoda-ku, Tokyo on my mobile device as you can see the screenshot below.
It's been 48 years to record the lowest temperature in Tokyo. It reached only 4℃ (39℉) during daytime. It is rare to have this cold temperature in central Tokyo.
January average temperature in central Tokyo is:
High 10℃ (50℉)
Low 2℃ (36℉)
In Tokyo, it is rare to have snow and barely have much snow. However, we sometimes experience train delay due to a bit of snow. You should be aware of taking public transportation under bad weather when you travel.
Have fun in Tokyo!