This is the 2182nd most frequent Japanese word.
毎月
"Every month."
Here, 毎月 specifies a regular obligation (rent payment) occurring at the end of each monthly cycle.
家賃は毎月の終わりに支払います。
The rent is paid at the end of every month.
In this example, 毎月 establishes the periodicity (once) over the span of a month, emphasizing the single recurring action.
健康のために毎月一回ジムに行っています。
For health reasons, I go to the gym once a month.
In this sentence, 毎月 indicates a recurring event (purchasing a book) that happens on a monthly basis.