The Division's public test server, the launch of which was delayed last week after Ubisoft decided that it wasn't quite ready for prime-time, is now set to open to the public on September 26. The new date was quietly revealed in the PTS FAQ, which also has information on how it will work and what you'll need to do to take part.
The PTS is a “different product” than the live version of The Division, and so you'll need to download the entire client again in order to access it. Your existing character will transfer over from the live server, although Ubisoft may sometimes request that PTS players create new characters for testing purposes, and your progress on the PTS will occasionally be overwritten by your live server character data.
The PTS itself will be available only “for as long as needed in order to get as much feedback as possible” on whatever issue is being chased down. Once that's done, it will be taken offline and remain so until new testing is required. A new PTS forum will open on September 22.
But before the test server can open, it must be tested too, and so a limited-access server technical test will be run from September 22-25. All players who registered for the Elite Task Force (that was the call for people to go to Sweden to help with the 1.4 update) will be invited to take part, and additional slots may be opened if it proves necessary. Ahead of all that, Ubisoft will stream an “exclusive PTS gameplay preview” tomorrow, at 12 pm EST/9 am PST, on Twitch.