The phrase "month in month out" refers to something that happens regularly and consistently every month. There are several synonyms that can be used to convey a similar meaning. One possible option is "consistently," which emphasizes the idea of something happening without fail or interruption. Another alternative is "regularly," which indicates a predictable pattern of occurrence. "Repeatedly" is another synonym that highlights the idea of something happening over and over again. Finally, "unfailingly" suggests a high degree of reliability and consistency, reinforcing the idea of something happening without fail or deviation.