Yes and no...there are crowd calendars, but none are universally thought of as being "good" or the "best". The ones most commonly mentioned are
Touring Plans (paid) and
IsItPacked (free). I'd take both with a grain of salt.
In terms of late September and early October, it really depends on when you are planning on going. Assuming 2015 is something like 2014, then HalloweenTime should be in full swing. It's a pretty popular "season", so I'd expect moderate-moderately high crowds on weekends and low-to-moderate crowds on weekdays. The first weekend in October is when Gay Days occurs and the second weekend is Columbus Day weekend (3-day weekend) plus CHOC walk (usually). Both of those tend to result in high crowds. By the end of September, MHP should have started, which usually occurs on Friday plus at least one other weekday (varies). We do not know if the parties will being DL or DCA this year.
I'd follow along with the
HalloweenTime SuperThread for more information about HalloweenTime.
I also have a
2015 Calendar of Events & School Breaks and DLR thread that tracks some of the bigger events that might impact crowds in DLR.