Buy the insurance, and buy it before a storm is named
September is the peak of the Atlantic season and the risk is real rather than notional. Central Florida storms rarely cause the parks to close for more than a day or two, but they routinely cause flight cancellations, and a two-day closure inside a five-day trip destroys the trip's value.
The mechanism to understand is named-storm coverage. Once a system has been named it becomes a known event and policies stop covering it, so a policy bought after a storm appears on the map does nothing for that storm. Buy at the time of booking.
Walt Disney World has historically operated a hurricane policy allowing cancellation without penalty when a warning is issued for the Orlando area or your point of origin within a defined window before arrival, and it covers the Disney-booked portion only. Confirm the current terms when you book, and choose an airline whose change policy is flexible rather than relying on that one.
The party ticket can be better value than a regular ticket day
On a September Not-So-Scary night, Magic Kingdom admission for party guests typically opens several hours before the party itself starts. Because the daytime crowd in September is already thin, that turns a party ticket into a long evening in a park that is quiet before the event and capped during it.
Run the comparison rather than assuming. A date-based single-day ticket in September is at its cheapest and a party ticket is also at the low end of its seasonal band, and depending on the night the party can deliver more usable hours per dollar than a standard day at the same park.
The reason this stops working in October is not the crowd but the price — party nights step up through the season, and the late-October nights are the most expensive of the run.
Check the refurbishment list — September is the second rehab window
January gets the reputation, but late September and early October are the other period when Disney takes attractions down, and it lands exactly in the quiet weeks that make the month attractive in the first place.
The practical implication is the same as in January: if the trip exists because one specific person wants one specific ride, check the closure list before booking rather than after. Shifting a September trip by two weeks is usually cheap and usually resolves it.
Water park closures follow a different logic — Walt Disney World has historically rotated its two water parks through seasonal refurbishment, so one of the two may be closed. If a water park day matters, verify which one is operating for your dates.
The weekday and weekend gap at EPCOT is the sharpest of any park-month pairing
The Food and Wine Festival draws a Florida-resident annual-passholder crowd that treats World Showcase as a Friday night out, and it produces the widest weekday-to-weekend swing anywhere at Walt Disney World. The same promenade can be walk-on quiet on a Tuesday afternoon and shoulder to shoulder on a Saturday evening.
The scheduling instruction is blunt: put EPCOT on a Tuesday or Wednesday and put Magic Kingdom or Hollywood Studios on the weekend, which is the opposite of what most itineraries do because people want the festival at night.
If a Friday or Saturday EPCOT visit is unavoidable, do the booths before 2 PM. The booths open later than the park does and are close to empty for their first two hours, and the whole crowd arrives after 5.
Disneyland is not on the same calendar this month
The advice that September is quiet is a Walt Disney World statement, and applying it to Anaheim will produce a bad trip. Halloween Time is installed across both parks, the seasonal ride overlays are running, and Oogie Boogie Bash nights are sold out, all of which keeps Disneyland's weekends at full fall attendance.
The version that does work is a midweek Disneyland trip. Tuesday through Thursday in September is soft at both parks, and it is the last easy stretch before the resort's Halloween and holiday seasons run continuously into January.
If the party matters and it has sold out, remember that the Halloween Time overlays are day-guest content requiring nothing beyond park admission. The party buys you trick-or-treating, character content, and a capped crowd, not access to the seasonal attractions themselves.