I can't tell you the time and date but start preparing your device for the update. Check out and follow the steps below.
Please take note of this, before you update you should follow these steps, so that you don't face issues such as data loss.
1. Please take complete backup of your phone using Smart Switch on computer.
2. Turn off Cloud Sync from Settings & Gallery if you take backup to Samsung Cloud and keep it turned off till beta testing continues. (mandatory)
3. You're now ready to use 'Install Now' button.
Tips for errors if they pop out after update :
You may encounter ''Android.process.media'' issue after updating the beta without above Settings.
So to fix this do this :
Go to Settings> Apps> Show system app" in the top right corner
A) Select "Media Storage" > storage > clear data
B) Select "sec media Storage" > storage > clear data
C) Select "Download Manager" > storage > clear data
You can bookmark this post you may need this after the update. Prevention is better than cure, so please don't ignore.