first Serpent day of the year
Refers to the first day of the Serpent (巳) in the traditional Japanese calendar, associated with certain customs and festivals.
On the first Serpent day of the year, there is a custom of visiting shrines associated with snakes.
From 初 (hatsu, 'first') + 巳 (mi, 'Serpent sign of the Chinese zodiac'). The exact historical origin of the associated customs is uncertain.