Today, 21st January is so-called “Daikan ( “major cold” / “大寒 ) , one of the solar terms.
A solar term, originated in China, then spread to Korea, Vietnam, and Japan
is any of 24 points in traditional East Asian lunisolar calendars
that matches a particular astronomical event or signifies some natural phenomenon.
Daikan is said that it is the coldest day in the year, placed in the middle of
Shokan (minor cold / 小寒) and Risshun (start of spring / 立春).