Koreisai is a collective term for shunkikoreisai (imperial ceremony of ancestor worship formerly held on the vernal equinox) and shukikoreisai (imperial ceremony of ancestor worship formerly held on the autumnal equinox). These festivals were established in 1878 by putting together the deathdays of successive emperors and major Imperial family members to worship them in the spring and the autumn.
The days for these festivals are now respectively called Vernal Equinox Day and Autumnal Equinox Day by the 'Act on National Holidays.'
A Tsuchinoe day that falls closest to the vernal or autumnal equinox day was originally known as a shanichi day, and shrine parishioners visited shrines which enshrined their local deities to pray for a good harvest on the spring shanich day or to thank for a successful harvest on the autumn shanichi day.
People worshipped a sobyo (a mausoleum containing the remains of one's ancestors) on this day in traditional China