also: おつや
noun
second watch of the night; around 9pm to 11pm
Archaic term from the traditional Chinese/Japanese timekeeping system dividing the night into five watches (五夜). 乙夜 is the second watch, roughly corresponding to 9pm–11pm. Not used in modern everyday language; encountered mainly in historical or literary contexts.
古典では、乙夜は夜の二番目の刻限とされる。
In classical texts, 乙夜 is considered the second period of the night.
From the Chinese system of dividing the night into five watches, each associated with one of the ten celestial stems (十干). 乙 is the second stem, hence the second watch. The exact derivation of the readings いつや and おつや is uncertain, but they reflect different on'yomi traditions for 乙.