Since August 1, 1590 (August 30, according to the Gregorian calendar) was believed to have been the day when Ieyasu TOKUGAWA officially entered the Edo-jo Castle for the first time, the Edo bakufu (the Japanese feudal government headed by a shogun) designated the day as the second most important holiday after the New Year's day.
また、徳川家康が天正18年8月1日（グレゴリオ暦1590年8月30日）に初めて公式に江戸城に入城したとされることから、江戸幕府はこの日を正月に次ぐ祝日としていた。