This year, the day of January 20 is called "Daikan (lit. big coldness)".
And the period between January 20 and February 3 is also called Daikan, too.
Daikan is the coldest day@(and term) of the year on the calendar.
This year, January 5 and the term from January 5 to January 19 is cold "Shokan(lit. small coldness).
Shokan is the day (and term) when the cold becomes severe in a year on the calendar.
We say that the term of Shokan and the term of Daikan together are Kanchu.
Kanchu is a very cold term in winter on the calendar.
The times of Shokan and Daokan are almost the same every year, sometimes being off by about one day depending on the year.
Graphs of the lowest and highest temperatures of daily in Nara from 2013 to 2017 are as follows.
It was a cold period around the time of Kanchu every year.
In 2019 Kanchu is January 6 to February 3.
Probably it will be a very cold period.
There is a word called "Sankan Shion".
It's originally a word for use in winter.
It seems that it is a word expressing that it is cold for 3 days and warm for 4 days to occur periodically due to the influence of Siberian high pressure in the northeastern part of China and the Korean Peninsula.
But in winter of Japan it does not happen much.
And during the changing from winter to spring, there are cold days and warm days, so the words are often used in those days.
(January 21, 2018)