We have re-scheduled the Moodle 3.9 upgrade for this Friday, June 18, at around 11:00 PM. There will be about 30 minutes in which Moodle will be unavailable, at some point between 11:00 PM and midnight.
For information about new features in Moodle 3.9, see https://docs.moodle.org/39/en/New_features.
There are also Youtube videos available.