二十四節気 (nijushi-sekki) is an old calendar which Japanese people have been using since Heian period (平安時代 794-1185) and originally created in China for planning agriculture around BC.770-BC.221.

In the ancient China, solar calendar was used for a daily life but they realised this system didn’t match the real season. In order to detect subtle changes of the season, ancient Chinese people divided a year into 24 according to the positions of the sun and this was introduced to develop agriculture.

Nijushi-sekki was registered as an Intangible Cultural Heritage in China by UNESCO in 2016.

What are the 24 seasons? Here is the list. The dates are for 2020 and they change every year.

Spring, Summer, Autumn and Winter are probably more common terms for us to describe the seasons. In Niju-shisekki, they are 立春 (Risshun), the first day of spring, 立夏 (Rikka), the beginning of summer, 立秋 (Risshuu), the beginning of autumn and 立冬 (Rittou), the first day of winter. These four days are called 四立 (Shiryuu) and are the beginning of each season. 

You may also be familiar to the vernal equinox (春分/shunbun), the summer solstice (夏至/geshi), the autumnal equinox (秋分/shuubun) and the winter solstice (冬至/touji). This division is called 二至二分 (nishi-nibun) and these were the base of the season. It was called in different name but the system was used in China more than 3.000 years ago.

Soon 大寒 (daikan) will start. Please keep yourself warm if you are in the Northern Hemisphere. 

References :

Nijushi-sekki:  http://chugokugo-script.net/koyomi/nijuushi-sekki.html

Nijushi-sekki: https://kotobank.jp/word/%E4%BA%8C%E5%8D%81%E5%9B%9B%E7%AF%80%E6%B0%97-109595#E3.83.96.E3.83.AA.E3.82.BF.E3.83.8B.E3.82.AB.E5.9B.BD.E9.9A.9B.E5.A4.A7.E7.99.BE.E7.A7.91.E4.BA.8B.E5.85.B8.20.E5.B0.8F.E9.A0.85.E7.9B.AE.E4.BA.8B.E5.85.B8

2020 Koyomi : https://eco.mtk.nao.ac.jp/koyomi/yoko/2020/rekiyou202.html