sixteen-day-old moon; moon on the sixteenth night of the lunar month
Refers to the moon on the night after the full moon (fifteenth night). In the lunar calendar, the moon rises later each night, so the 十六夜 moon appears later than the full moon. Often used in poetic or traditional contexts.
The sixteen-day-old moon is a slightly waning moon seen the day after the full moon.
The moon on the sixteenth night of the eighth month in the lunar calendar is called 'izayoi'.
The reading いざよい is a native Japanese word. The kanji 十六夜 literally mean 'sixteen night', matching the meaning. The historical derivation of the reading is uncertain.