Can't comment on Labor Day itself, but we went to MK the day after Labor Day last year and had no wait over 2 minutes for anything except Space Mountain (5 minutes), meeting Mickey in Toontown (5 minutes), Jungle Cruise (5 minutes), food at Cosmic Ray's (5 minutes), and Haunted Mansion (10 minutes). Nearly anywhere there was a daunting line, FastPass was available and worked, the exceptions being AstroOrbiter (pretty much always a line) and IndySpeedway (ditto). We did everything we wanted and at our own pace, no rushing at all. Same thing the next day at D-MGM-- longest wait was 5 minutes for RockRollercoaster. Wait for Tower-- 0 minutes.
With Labor Day a few days later this year and closer to many schools' fall breaks, it will probably be even less crowded than last year, although they are offering a lot of resort deals to lure folks in.
The main thing-- use the Fast Pass. The wait for Peter Pan* was near 30 minutes for those without FP in late afternoon, but we waited maybe 2-3 minutes in the FP line.
We are going earlier this year (last of August) and hoping the crowds won't be too much bigger, as we are now spoiled by zero waiting times
*edit--originally wrote "Snow White", but realized it was "Peter Pan"