A year was divided into 24 solar terms on the traditional Japanese calendar. Shubun, or Autumnal Equinox, is the 16th solar term. It usually begins around September 23rd, when the Sun reaches the celestial longitude of 180°. Autumnal equinox in astronomy is the moment when the Sun is at the equinoxtical point. Night and day are of nearly the same length at an equinox both in spring and autumn.

Spring and Autunmal Equinoxes are national holidays in Japan, for people hold a memorial service for their ancestors and dear deprted family members on these days. Originally, the autumnal equinox was the day to hold a Shinto ritual to thank the god for rich harvest and return the god to the mountain, who had came down to the village to guard the harvest. With the spread of Buddhism, it took on the meaning of service for one’s ancestors’ souls.

Since autunmal equinox became Imperial Memorial Day in the Meiji period (1868-1912), it became the custom for general people,too, to hold a memorial service on this day.