five-period night division
Archaic system dividing the night (roughly 7pm–5am) into five equal two-hour periods. Used in historical contexts.
In the Edo period, the night was divided into five periods to tell the time.
fifth night period (approx. 3am–5am)
Specifically the last of the five periods, corresponding to the pre-dawn hours. Also archaic.
The fifth night period was considered the darkest time before dawn.
night of the full moon; 15th night of the 8th lunar month
From 五 (five) + 夜 (night). The term reflects an old Chinese-influenced timekeeping system adopted in Japan, where the night was divided into five equal periods.