We were there last Labor Day and the parks are busy but not as bad as other holidays. It does tend to get slow the Tuesday after Labor Day as everyone has to head back since kids have to be in school.
A lot of people within close driving may come down for the weekend but since alot of kids start school before the holiday it's not too bad as a lot of people won't fly down for just a couple of days.
MK will tend to be the busiest and Epcot should be fine as it handles larger crowds easier.
We just got back yesterday and the crowds at Epcot were not bad for it being the 4th of July so I am sure Labor Day will be fine.
