We (Brits) were in on Labor (Labour) Day weekend last year and were astonished just how quiet it was - the explanation we were given was that this was a family picnic weekend and the park was traditionally quieter
The '05 Member's Cruise began on the Sunday of Labor Day weekend, and what I remember most from the day before, which we spent at WDW, was not the heat, but the rain, it poured, and not just a 10 minute shower! We spent the day after the MC at MGM and it indeed was a quiet day, we got to do everything we wanted and walked in to get a table for 2 at the Brown Derby, no waiting!
Yes, along with the heat and humidity comes torential rains sometimes, and even hurricanes! The last Labor Day we were there was 2003, and the rain was pretty much non-stop all week.