day of the Snake
Archaic term referring to the day of the Snake in the traditional Japanese sexagenary cycle. Now mostly encountered in historical or cultural contexts.
In the old calendar, special events were held on the day of the Snake.
From 巳 (mi, 'Snake', the sixth sign of the Chinese zodiac) + の (no, possessive particle) + 日 (hi, 'day'). The reading みのひ is a straightforward combination.