Archaic poetic term for the full moon, especially the mid-autumn harvest moon. Not used in modern everyday speech.
In classical literature, '最中の月' sometimes refers to the mid-autumn harvest moon.
Specifically the mid-autumn harvest moon, a common seasonal reference.
From 最中 (もなか, 'zenith, height') + の + 月 (つき, 'moon'), literally 'moon at its height'. The exact historical derivation is uncertain, but it is an old poetic expression.