The middle of the month, around the 15th day.
Midmonth is a time when people might get paid or when bills are due.
Examples:
We get paid at midmonth.
My birthday is in midmonth.
We have a party at midmonth.
At midmonth, we eat a lot of cake.
In midmonth, the sun is very hot.
At midmonth, we have a big celebration.