According to a teacher at one of my schools, 19th January was "Daikan," 大寒 ("big cold"), and was supposedly the coldest day of the year. Ten days later and having repaired my cracked shower head no less than three times due to it freezing over night, I'm going to disagree with that speculation. Last years coldest point was early-mid February, and due to this year's winter being far more severe, I'll further speculate that I'm going to find more than just my shower sponge frozen solid in the mornings to come.
According to another teacher at a different school, 4th February is "Rishun," 立春 (first day of spring). I can't say I have much faith in this named day either. It is bitterly, bitterly cold in Nagano prefecture at the moment and every night falls very uncomfortably below 0 degrees. For the past two weeks I've been making my breakfast in the kitchen at a balmy -4 degrees, which wakes you up something awful. Amusingly, I wake in the morning to a bedroom temperature of about freezing, maybe a couple of degrees under, scuttle into the kitchen to make breakfast, then scuttle back again, only to find the bedroom temperature rather acceptable in comparison. Small mercies, and all that. It doesn't stop me using my glorious kerosene heater, though.