Generally, Oracle releases monthly updates on the first Friday of the month to all test environments. Oracle applies the update to test environments during the next daily maintenance after the release of the update.
Typically, production environments are patched on the third Friday of the month. See Understanding Updates to an Environment and Viewing Readiness Information.
The Enterprise Performance Management page of Oracle Cloud Readiness provides easy access to documents describing the new features included in the monthly update. It also identifieds the update schedule for test and production environments. Additionally, Oracle notifies all Service Administrators of upcoming updates through an email, which is sent on the last Friday of the month (one week before the Test environment is updated).
Monthly update notifications will still be sent to the Service Administrators of the environments for which upgrade has been put on hold.