The week after Labour Day is traditionally one of the lowest crowd levels of the year. This year will be impacted by Galaxy's Edge Phase 1 opening the week prior, but nobody knows for sure how much of an impact. My guess is that Hollywood Studios will see an uptick for sure, but I don't think the overall resorts and other parks will be significantly different than they usually are for that time of year.
Since it's the only day you can go, then set your expectations for moderate to high crowds and if you get the usual low to moderate, then you'll be pleasantly surprised. Even if you don't have tickets yet and can't pre-book fastpasses, you should still take advantage of booking them day of there will lots of availability for most rides.