That's a great question. You put a heading of when is the best time of the year to visit Disney World which is a whole different question from when is the least crowded. I tell you what. I'll answer both.
Now as to the slowest times, that would be from mid-January to mid-February or September. Those are the slowest times at Walt Disney World. When it comes to avoiding crowds early in the year, from the week after the Martin Luther King holiday to President's Day weekend are just about as slow as it gets. Here are the crowd calendars from January and February.