On a percentile scale, the crowd level is in the lowest 10% or 20th percentile on those days.
Meaning that the total number of people visiting on that day is among the lowest 10% considering what that number is on other days of the year.
Basically late September is less crowded than October. But September has worse weather than October. It still has more potentially of being uncomfortably hot and there is on average more than twice the rainfall in September than October.
The height of hurricane season is Sept 1, which doesn't mean a lot for Orlando, but it does mean that August and September see more tropical rain style weather. By October, things are cooling off and settling down comparatively.
Average rainfall in September is 14 days out of 30, and the average for the whole month is 5.8 inches. That comes out to .4" per day that it rains. But our experience with September is with storms that dump a bit of rain and lasted for several hours. October has rain statistics of significantly lower rain amounts.
If you were to pick a week on the crowd calendar that is the least crowded, one of the lowest would be September 18 to September 24. And 9/22 through 9/24 are considered to be less than the 10th percentile of crowd days. So, 90% of the time it would be more crowded than those days. Or conversely, you can only go on less crowded days 10% of the time compared with Thursday, Friday, and Saturday of that week.
If you mean crowd size at MNSSHP on a Friday in September, then you could safely say that they come close to selling out but don't always sell out. But selling out is a significantly lower crowd level than a crowded day, but it is more crowded than a day when MK seems like a ghost town. And one thing about the MNSSHP, people all crowd on the parade route and want to watch the special fireworks and castle shows. So, it is really crowded along the parade route in the "hub" area just north of the main st where it's flanked by stores.
I think MNSSHP is probably more crowded on Friday than Tuesday in September and early October. But they're both pretty equal. Then it's really crowded for late October i.e. Sun 10/30, Monday 10/31 and Tuesday 11/1.
So, if you're arriving on a Friday, don't pre-purchase your MNSSHP tix for that night.
I would just see how the travel day goes. If everything is smooth and you're not that tired, you could purchase tickets at the gate and go. But most likely, you'll want to go to MNSSHP on the following Tuesday given that you're arriving on a Friday.