Although Twitter can be unreliable for such information (and that tweet is now gone), I can confirm that the August 9 date is accurate. Nevertheless, there is more nuance required.
Tuesday, August 9th is the planned release date for the Windows 10 Mobile Anniversary Update for open-market Windows 10 Mobile devices e.g. unlocked Lumias and presumably Coship, NuAns, Acer, and more.
The previously mentioned nuance is for mobile operator branded phones sold through carriers e.g. Lumia 950 on AT&T and Alcatel OneTouch Fierce XL on T-Mobile. These phones are planned for a Tuesday, August 16th release one week later.
The reason for the one-week separation is to give carriers some extra time for testing “features and enhancements” from the update, which is typical. Considering most Windows 10 Mobile devices are open-market these days the majority of phones should get the Anniversary Update around August 9.
Of course, both dates are subject to change should any last minute problems be discovered before general release. Nonetheless, Microsoft is not expecting any delays at this point and both times as of today seem to be a lock.
While many open-market Lumia 950 and Lumia 950 XL phones have already grabbed the 16236 firmware announced in July the AT&T Lumia 950 has yet to receive it. If I had to guess, AT&T would co-release both the new firmware and the Anniversary Update as a package on Tuesday, August 16. However, we’ll just have to wait and see. That firmware delivers double-tap to wake, fixes for Wi-Fi and improvements to the PureView camera.
As far as the final build that will be pushed it could 14393.10. While that build is not yet released for Insiders a recent changelog for the PC variant did mention a fix specifically for mobile:
Addressed issues that cause high battery drain when a Windows 10 Mobile device is idle or the screen is off.
That modification implies that an update is yet to come. Otherwise, Microsoft could just ship 14393.5, which is the latest release for all Insider rings.
We also have some late information that suggests 14393.67 could be the final build pushed fo the Anniversary Update.
Source : WindowsCentral