also: おつや
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.
In classical texts, 乙夜 is considered the second period of the night.
Archaic term; the reading いつや is the standard on'yomi for this compound.
Alternate reading; おつ is a less common on'yomi for 乙, but appears in some historical contexts.
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.