Being kind of a local weather hobbyist, thought I'd chime in on this one.
Climatologically speaking, September is the wettest month of the year for southeastern PA with a 4.45" average. October ranks 5th at 3.70". So moving the date a couple of weeks would have no real impact from a long term perspective.
I've been attending the Keystones since 1996 and it does indeed seem like a run of weather bad luck, especially the past several years or so. Last year was almost comical with at least some rain affecting the ADRL, Super Chevy, Rumble, Nationals and the Dutch events.
But if you think about it, it takes an almost unusual situation to get through an entire event with no weather influence in this part of the country. In the warm season, we average basically 1" rain/week. So we don't want a heavy event early in the week since that affects parking and obviously none during the event itself. There's another 3 or 4 days. So what's left? Zip.
So couple the fact that Maple Grove requires a lot of grass parking, does not drain for crap and usually sees precip several times/week on average, it's inevitable the weather will have negative impacts more often than not.
And for the next ten days, I'll say no rain Friday, a sprinkle at worst on Saturday, Sunday too close to call (literally 30 miles will make the difference depending on where the rain axis sets up) and the Dutch will be close to perfect all 4 days
(remember, this is 10 days in advance so no harrassment if we have to build an ark next Friday).