The traditional Japanese calendar tends to divide the calendar year into 24 parts, reflecting the changes of the season and so on. One of the more memorable dates is something called daikan (大寒) or “great cold” as well as the lesser-known shōkan (小寒) or “little cold”. If you look on a Japanese calendar, especially one of the more traditional ones, you’ll see both of these on there, about 2 weeks apart. Daikan is supposed to mark the coldest day of the year, January 20th in modern calendars, however it also marks the period from that day until traditional coming of Spring, or risshun (立春) which in this context is February 3rd.
In reality, daikan doesn’t always reflect that actual coldest period of the calendar year since it was based on the Lunar calendar which shifted around, but also just due to local weather variations (to say nothing of climate change 😅). Further, the 24 parts of the traditional calendar are originally from China (particularly the Yellow River region) and imported into Japan, and since both had differences in climate and weather, not everything translated 1:1.
Speaking of the 24 parts, the date of shōkan is two weeks before daikan, so January 6th, but like daikan, it reflects not just the date, but the intervening period until daikan.
However, daikan/shōkan were still useful culturally because they expressed the sense of deepening Winter before the coming of Spring. Shortly after New Year, the weather starts to get even colder (shōkan), and colder (daikan) until it finally turns the corner and the first signs of Spring appear (risshun).
One other interesting note is something called daikan-tamago (大寒卵). It was traditionally thought that eggs laid on or during daikan would confer good health to people who ate them, especially children. Because chickens back then were raised more in outdoor coops, their egg-laying cycle wasn’t always year-round. They might lay fewer eggs in the deep of winter hence the eggs were more special.
Anyhow, if you’re in the Northern hemisphere, stay warm and remember to enjoy a daikan-tamago if you can. ;p
Big thanks to jpnculture.net for the helpful information!