When Microsoft had announced Windows 10 Build 16237, they had announced that they will be starting the 2nd and the final Bug Bash for Fall Creators update from 12am (Pacific Time) on Friday July 14th that will run a full weekend ending at 11:59 pm (Pacific Time) on Sunday July 23rd.
Today, Microsoft had announced that the first of two bug bash webcasts will be broadcasted on Mixer page for Windows Insiders. It will start at 1:30pm PDT, 8:30pm UTC. There will be another webcast for this final bugbash for the Fall Creators Update.The second webcast will start on July 21st. The bug bash webcast will allow Windows Insiders to interact with the Windows Insider during the period of the bugbash.
The Bug Bash is basically a series of quests(tasks, rather) aimed at dedicated insiders to find out the bugs and discrepancies in Windows 10 insider builds. They are primarily aimed before major feature updates for Windows 10, so that the build pushed out to the general public is free from annoying bugs.
The Engineering team during this period will post several Quests for Windows 10 Insiders to be looking through and then sharing their Feedback. The feedback submitted will then be reviewed by the relevant Engineering teams to resolve as many issues as possible. The fixes will then be rolled out in the next Windows 10 Insider build which will be a bug fixing update.