Many people on SNS have noted that Setsubun, the eve of risshun (立春), or the first day of spring according to the lunisolar calendar (tai'in/taiyō reki, 太陰太陰暦), fell on February 2nd for the first time in 124 years. The last time this happened was Meiji 30 or 1897.

But why is this?

Like many “uniquely" Japanese traditions, risshun is actually one of the 24 solar terms of the Asian calendar which originated in China, but spread throughout the East Asian cultural sphere. In China, it is known as Lìchūn and begins when the Sun reaches the “celestial longitude of 315° and ends when it reaches the longitude of 330°. It should be noted that each of the 24 solar terms are spaced 15° apart along the ecliptic, or the plane of Earth’s orbit around the Sun.

It takes the Earth 365.2422 days to orbit the Sun and because of that 6 hour difference, the exact time at which the four annual setsubun (yes, four) occurs differs from year to year. Today, we add one extra day to the year every four years, or every leap year. The leap year in Japan is called urūdoshi (閏年). Before the adoption of the Gregorian calendar, each month in Japan lasted 30 days, with the year lasting only 354 days. In order to make up for the loss of 11 days a year, an urūtsuki, or “leap month”, was added every three years.

Incidentally, each solar term is divided into three pentads, or (候, hòu in Chinese; kō in Japanese), meaning there are actually 72 pentads or “micro seasons” in Japan, rather than the exalted four. The present micro season or kō is Harukaze Kōri-wo Toku (東風解凍, also tōfū kaitō) and means “Spring (or Eastern) winds thaw the ice."

The next solar term is Usui (雨水, yǔshuǐ in Chinese, ushii in Ryūkyūan) will fall on February 19th and refers to spring rain.

If you would like to know before hand what day Setsubun will fall on, divide the year by four. From 2021 to 2057, if 1 remains, Setsubun will fall on the 2nd; if two, three, or zero remains, Setsubun will fall on the 3rd. Because of this Setsubun will fall on the second of February every four years from 2021 to 2057.