Oracle periodically performs changes to cloud hardware infrastructure, operating software, product software, and supporting application software to maintain operational stability, availability, security, performance, and currency of your applications. In order to perform these changes, Oracle reserves specific maintenance periods for changes that may require Cloud Services to be unavailable.
For details, see Global Business Unit (GBU) Cloud Quarterly and Monthly Maintenance Schedule (Doc ID 1991668.1) on My Oracle Support at http://support.oracle.com.