Bethesda have announced the starting dates for PC, PlayStation 4 and Xbox One. Microsoft's console will get the beta first, on 23 October while PC and PlayStation 4 will get to jump into West Virginia a week later, on 30 October 2018.
Microsoft appears to have struck a deal with Bethesda since "technological difficulties" weren't quoted anywhere and Bethesda even advertised Xbox One getting access first on the in-engine intro they posted earlier.
Considering that the beta will be exclusive to a pool of players chosen from the fans who pre-ordered Fallout 76, the time exclusive deal appears to be discriminatory towards PC and PlayStation 4 users. They all shelled out money up front, and now they have to wait an entire week because Bethesda seemingly appreciates it more if the game was purchased on Microsoft's store.
According to the official post, the one of the main purposes of this beta will be to stress test the servers. Due to the nature of the test, it will run only during specific times, most likely peak hours and therefore the beta servers will not run 24/7. That said, the game will be up for four to eight hours a day, but there is currently no schedule on when those days might be either.
There is a second goal of the beta that Bethesda touched upon, which is to prompt players to break the game in any way possible. If Bethesda's earlier Fallout or Elder Scrolls releases are any indication, bug hunting enthusiasts will have a field day.
Smaller stress tests are apparently already underway but these are reserved for a smaller group of randomly selected players who have signed an NDA, so while earlier gameplay leaks are possible, they are not likely. If you're craving some more Fallout 76 footage, Bethesda announced plenty of gameplay, guides, tips and tricks videos in the upcoming weeks.