Using TouringPlans statistics and info, here are what the avg crowds (scale of 1 - 10, 1 being empty, 10 being insanely packed) were per month last year and up thru yesterday for this year:
2017
5.70 - January
6.00 - February
7.55 - March
7.13 - April
6.50 - May
6.77 - June
6.65 - July
6.48 - August
4.14 - September
6.16 - October
6.37 - November
7.61 - December
(6.42 - Avg crowd for 2018)
2018
6.67 - January
6.79 - February
6.90 - March
5.10 - April
5.45 - May
7.33 - June
5.35 - July
5.00 - August
(6.07 - Avg crowd for 2017)
Using those statistics, and if this most recent July and August were any indication of what crowds we see this upcoming September (assuming we see the same type of drop-off that we saw from 2017-2018 occur), crowd levels this Sep will probably be around a daily avg of about 3.23 this year for the month compared to last year's 4.14.
Anyways, it's clear as day if someone looks at 2017 that September is by far the slowest month of the year still at WDW and by a pretty fair amount. And it only makes sense, since that is when school starts for pretty much everyone all over.