Seasonal days (zassetsu)

Solar terms are the moments the Sun’s apparent ecliptic longitude reaches multiples of 15°, computed with Skyfield and the JPL DE421 ephemeris and given in Japan Standard Time. Old-calendar months begin on the JST date of each new moon; the month containing the winter solstice is month 11, and in a year with 13 months the first month without a principal term is the leap month. Rokuyō follow (month + day) of the old calendar; day signs follow the 60-day cycle. Seasonal days use the conventional definitions (e.g. setsubun = the day before risshun, hachijū-hachiya = the 88th day counting risshun as day 1). Dates were cross-checked against the National Astronomical Observatory of Japan’s published calendar.