The very lowest crowd levels - IMHO is between 2-3rd week of September 17th-22nd. If you arrrive on a Sunday and leave on a Friday, the ride waits are minimal. There are a few reasons for this:
1. Kids have JUST returned back to school. Tha parents that may take kids out of school for a WDW vacation will not do it at this time.
2. It is peak Hurricane season - Actually Sept 14th is peak "hurricane day" Why that particular day I don't know.
3. If you go Sun-Fri (sept 17-22), it is between
MNSSHP dates and before the Food and wine festival starts. Also any "new" celebrations usually start Oct 1st. It is actually the "doldrums" of the WDW calendar...nothing going on. Since locals visit the parks on the weekends, you are avoiding that by arriving on a Sun and depart on Fri. Most others are done with their vacations.
4. Free Dining has no impact on crowd levels. It may only make it more difficult to get an ADR at certain TS restaurants like Le Cellier.
If you were going to go to WDW during Sept - Oct, most people would wait till something is actually going on.
This is JMHO and obeservations from going every September. It is even slower than going after Thanksgiving week thru the 2nd week of December.