We already know a lot about the Windows 10 Fall Creators Update. The company announced the release at Build 2017 and has been sending out early features to Windows Insiders. We already know the update will bring features such as security improvements and Fluent Design. However, we still do not know the codename version number.
Microsoft also said during the introduction that it is targeting a September 2017 release for the Fall Creators Update. That makes a lot of sense considering the company is committed to two Windows 10 releases per year.
A new official document shows that the next major Windows 10 release will be codenamed version 1709. Again, this makes a lot of sense.
Microsoft numbers its build codenames by year and month. This can be seen with the current Creators Update, which is also known as build 1703 (2017 and the third month.). Yes, it was release in April (04), but it was finalized in March.
— WZor (@WZorNET) 3 July 2017
So, the Fall Creators Update will be build 1709, 2017 on the ninth month. This means Microsoft's September timeframe for the release of the update is finalized. Sure, we could get a Creators Update scenario where the Fall release is finalized in September, but launched in October.
Either way, it would be early October at the latest (the Creators Update dropped on April 11).
It is worth noting that the final launch month/date of the Windows 10 Fall Creators Update does not mean a lot for when users actually get it. Microsoft rolls out its OS upgrades in stages. The Creators Update is still making its way to machines a few months after release.
This means some users may not actually get the Fall Creators Update until much later in the year.
As always, Windows Insider Program members will be getting the RTM (release to manufacturing build) before anyone else. Those users will almost certainly see the finalized build during September.