A monthly session keeps its first day of the month. One that starts on the 31st only happens in months with a 31st.
Sessions
Creating sessions
Set up trainings, shifts and events that repeat on a schedule.
Sessions live in the Activity module, under Activity › Sessions.
A session type is a repeating event, such as Training every Saturday at 7 PM. Create one with a name, its first date, a start time, and how often it repeats: once, daily, weekly, every two weeks, or monthly.

