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.
I lived in Miami for over 35 years and IMO the best time to travel to Miami is in the beginning of October to mid November because of less heat/humidity and less crowds. This is also true for Ft. lauderdale and West Palm Beach - heck....all of Florida 😉That helps you enjoy your visit out doors and the theme parks if you wish to go to Orlando.
Also, if you are going out of Miami, Ft. Lauderdale, Orlando or Tampa on a cruise - its one of the best times for you to get better cabin caterogies for balconies, suites and butler suites at lower investment - sometimes up to 40% less!