According to the classical encyclopedia "Shukaisho," 'Hyakki-yako days,' the days when a hyakki yako appears, were on the New Year, the day of the Rat in February, the day of the Horse in March and April, the day of the Snake in May and June, the day of the Dog in July and August, the day of the Ram in September and October, and the day of the Dragon in November and December (old calendar), so people used to avoid going out at night on those days because it was said that they would die if they came across a hyakki yako.
古典の百科全書『拾芥抄』によれば、正月、2月子日、3月・4月午日、5月・6月巳日、7月・8月戌日、9月・10月未日、11月・12月辰日は百鬼夜行が出現する「百鬼夜行日」であり、百鬼夜行に出遭うと死んでしまうといわれたため、これらの日には夜の外出を控えたという。