The slowest month in Miami - at least in terms of tourist numbers - is in September.
September represents the time during which hurricanes are most likely to make landfall in the neighbouring Caribbean or even in Florida. I was present during Hurricane Irma!
Schools are out for summer further north, while children in Florida return during August.
Some in the travel season call September in Miami easy season. Meaning life gets a bit easy as there are far, far fewer out of state visitors.