Definitely October.....
I'll use your request for two purposes: to answer your question first by offering my favorite time to be there,
but also I'll use this opportunity to plug Williamsburg in general for anyone else interested in a great Fall destination.
Hey, the beach stuff is over, the heat is gone, and altho it was fantastic and much fun for the family, our hearts are elsewhere!
Now it's the time for what we truly live for----FALL!
And for Williamsburg I'd recommend only starting around
mid-October because it's cooler and more comfortable, and the foliage is more beautiful,
peaking then and extending into November.
It's really a glorious time to be there, whether it's in Wmsbg or anywhere else in that part of Virginia.
You mentioned September, but I'd even prefer November over September....cooler and still beautiful foliage, and very nice temps to get outside and explore.
We now visit Williamsburg
every year, specifically staying at Manor Club, and it's a place we've fallen in love with more and more every time we stay.
We own there because we love to make annual stays, but it also is a great trading resort.....
And even though we trade, we still stay there at least one full week a year by whatever means we can.
(BTW, Marriott's Williamsburg Lodge is a fantastic hotel which is available using Bonvoy Points, if you decide to go that route,
and it's easily walkable to all of the Colonial Wmsbg things I'll mention here.
https://www.marriott.com/en-us/hotels/phfak-williamsburg-lodge-autograph-collection/photos/ )
There's a lot to do in and around Wmsbg and we never tire of it. Since it's beautiful in Fall, our greatest desire is to go on walks and enjoy the outdoors,
and the trails around Manor Club (clean, well-kept asphalt paths), especially alongside the golf course, are wonderful.
There's also the Premium Outlet Mall within a very short drive (10 min) which is awesome----good for a half-day or full day, whatever, with lots of great stores. It's large!
Colonial Wmsbg is a fun place to just walk around, regardless of whether you're purchasing and using the pass that gives access to the historic buildings and venues.
But you can still walk around the Colonial Wmsbg town and enjoy it
without the pass, and there are many restaurants there on site,
several which are on my personal "favorites" list. Merchants Square, adjacent to Colonial Wmsbg, is exceptional for shopping, dining, or just hanging out.
And with the approaching holidays, it's always decorated for the season.
Not to miss: The College of William & Mary "across the street" has a gorgeous campus on which to walk around and simply enjoy being out.
Awesome and historic place!
Colonial Wmsbg, Merchants Square, William & Mary campus, several worthwhile museums, and more ....... all are easily walkable.
We've been known to sit on a lovely park bench or two for hours (they're scattered around everywhere), just to people-watch and have a little "down time".
Merchants Square is a remarkable example of American design with over 40 shops & restaurants. Find unique experiences in a setting of unrivaled beauty & charm.
www.merchantssquare.org
About every other year we make the 2-hour drive to Charlottesville to visit Monticello. There are also many other historical sites to visit,
including Jamestown & Yorktown, or one of several of the nearby plantations such as Berkeley Plantation or Shirley Plantation, to name only two.
I mention these things because this time of year, especially October - November, is the most comfortable and beautiful time to visit.
It's not crowded, and yet it's such a wonderful season.
And October is definitely the month.