first watch of the night (approx. 7pm to 9pm)
Archaic timekeeping term from the traditional Chinese-influenced system dividing the night into five watches. Not used in modern everyday Japanese.
In the old time system, 甲夜 corresponds to the first period of the night.
From the traditional Chinese system of dividing the night into five equal watches (五夜). 甲 (first of the ten celestial stems) is used to label the first watch.