day of the Boar
Refers to the day of the Boar in the traditional Japanese sexagenary cycle. Associated with certain customs, such as eating 亥の子餅 (inoko mochi) for good health.
亥の日には亥の子餅を食べる風習がある。
There is a custom of eating inoko mochi on the day of the Boar.
From 亥 (i, 'Boar', the twelfth sign of the Chinese zodiac) + の (no, possessive particle) + 日 (hi, 'day'). The term is deeply rooted in the traditional calendar and associated folk customs.