Consider the time of year. Disney World is most packed on major holidays like Independence Day, Easter, Memorial and Labor Days and, to a large extent, all of spring and summer school breaks. Ironically, November, December and January are the park's lightest months ... that is, when it's not hosting major celebrations for Thanksgiving, Christmas and New Year's. The week of President's Day (in February) is also very crowded. Your best bet is the window between the end of Thanksgiving break until a week before Christmas.
Consider time of day. Even if you have to visit Disney World during its peak season, you can cut down on crowds by joining the ticket line early; park hours are usually extended during the crowded seasons just to give visitors a little more breathing room.
Remember that Disney World is a tourist destination, so visiting during a weekday will not necessarily offer you the same low-attendance benefits as visiting an attraction in your local area on a weekday. Since families are already on vacation, school and work schedules don't prevent visitors from spending a Monday or Wednesday taking in the park. That said, as a rule of thumb, Disney World is often uncrowded when school is in session and families are less likely to interrupt kids' schedules with a vacation.
