The least busy times at Disney World are mid January to early February, late April, and early September through mid November. These times fall outside typical school break times.
Slower times at Disney World may also mean lower rates for hotel rooms and other attractions. Orlando in general tends to have fewer visitors during those slower months so families have shorter waits at other attractions.
While general time periods see fewer visitors to Disney World, some days within the range are still busy. Special events such as Mickey's Not-So-Scary Halloween in October and the runDisney events in January draw large crowds. Disney World also tends to offer special promotions during less busy times, potentially increasing attendance.