Got some friends who are heading to the island in mid September and they have asked me to compile a list of places to see, do and visit. Anyone know of some regular places that will be closed for low season?

I am,

But your heart is always open Elbert.

Carb, my son and I were there last Sept and Estelle's was even closed!! A number of places were, but strangely enough we found enough to eat and DRINK!!!

Aw, heck. I'm arriving 9-10! Bummer!

Legends burger joint is closed now???

Yes, Legends is closed.

Seems Casa Picasso is also closed for slow season. Regrettably it appears slow season is beginning earlier each year. Certainly understand it from a business owners perspective. Feel sad for the employees who are out a job for up to 3 months though. Just more of a reason to support those businesses that do stay open and slog it out.

Tranquility Bay Resort and the Aquarium Restaurant will be closed for September.

One of the reasons for closing in September is that many employees have family homes that are off-island, and they wish to return to them for the duration of the September Celebrations. It works well for establishments to give employees their paid vacations at a time when they can enjoy them, and at a time when the rate of visitor arrivals is at a natural low point in the year.
Further as Elbert points out so well in his graph, the risk of storms is high and again people like to be "home" when weather threatens. For those owners and workers who are still in the area during September, it's a good time to do annual cleaning, repairs etc all at one time.
This is a seasonal business, no doubt about it.