noun
night of the full moon; 15th night of the 8th lunar month
Refers specifically to the full-moon night of the 15th day of the 8th lunar month (around mid-September), a traditional moon-viewing festival. Often associated with offering susuki grass and dango.
今年の十五夜は九月十五日です。
This year's harvest moon night is on September 15th.
十五夜にはすすきを飾ります。
We decorate with pampas grass on the night of the full moon.
From 十五 (fifteen) + 夜 (night), referring to the 15th night of the 8th month in the old lunar calendar.