noun
new moon; invisible moon
Rare, poetic term for the new moon, specifically the phase when the moon is completely invisible. More commonly expressed as 新月 (しんげつ) or 朔 (さく).
三十日月の夜は、空が真っ暗で星がよく見える。
On the night of the new moon, the sky is pitch black and the stars are clearly visible.
Compound of 三十日 (みそか, 'last day of the month') and 月 (つき, 'moon'), referring to the moon on the thirtieth day of the lunar month, when it is invisible. The exact historical derivation is uncertain.