Today is Daikan, literally 'big cold', historically the coldest day of the Japanese calendar, when the sun is at a celestial longitude of 300 degrees. (I may need to have a chat to a friendly science teacher to understand exactly what this means.) The daikan period goes from the 20th Jan, for two weeks, until the equinox. (There 24 mini seasons in the traditional Chinese /Japanese calendar. This website shows them well. )
Today was anything but a daikan. Across Japan the weather was a historical aberation; in the northern city of Sapporo, where ice skating rinks were melting, according to www.wunderground.com, today was a record hot at +5. The average daikan temperature in Sapporo on 20 Jan. is -2.
Throughout the country it was much the same - record high temperatures everywhere.
Akita average 1 today 10 - a record (Japan seaside - north)
Niigata average 5 today 11 - a record (Japan seaside - middle)
Morioka average 2 today 7 - a record (Pacific side north)
Tokyo average 9 today 18 - a record (Pacific side middle)
Osaka average 9 today 15 - a record (inland sea side)
Fukuoka average 10 today 19 - a record (coastal Kyushu,facing Korea)
As much as I dislike winter, there is a secure feeling when weather acts in accordance with recent historic averages.
This page of wikipedia uses the Chinese names, which in many cases are very similar. http://en.wikipedia.org/wiki/Daikan