noun
13th night of the lunar month; moon-viewing on the 13th night of the 9th lunar month
Refers to the 13th day of a lunar month, especially the traditional moon-viewing custom on the 13th night of the 9th lunar month (around October). Often paired with 十五夜 (the 15th-night moon-viewing).
今年の十三夜は曇りで月が見えなかった。
This year's 13th-night moon-viewing was cloudy and we couldn't see the moon.
十五夜と十三夜の両方で月見をするのが縁起
It is said to be auspicious to do moon-viewing on both the 15th night and the 13th night.