Salesforce Summer 18 Sandbox preview started on May 4th. Current plans show that Salesforce Summer 18 will be rolled out to production instances June 1st and June 8th.
Details can be found on pages 5 - 6
Want to know what version of Salesforce you're using? Using the App Launcher in the upper right hand corner, change to something other than BMC Remedyforce and look at the icon in the upper left hand corner. If you see this:
Then your Sandbox is on Salesforce Summer 18. Please be aware your results may vary depending on how your org is configured.
URL Format Change
Salesforce will be changing the URL format used by Lightning Experience standard apps, console apps, and the Salesforce mobile app. If you have customizations that parse URLs, those customizations could break. There is a new Critical Update that's available today that you can enable and test within your sandbox.
See Page 7 for full details.
Full Content Push Notifications
With Salesforce Summer 18 release, all push notifications to v16.0+ of the Salesforce mobile apps for iOS and Android will be sent fully encrypted.
Details on page 9.
AP9 on May 19, 2018 15:00 - 16:00 UTC
AP9 on June 2, 2018 15:00 - 16:00 UTC
Details on pages 13 - 15, with a more complete calendar on Page 15.
Salesforce Maintenance Schedule
See page 16 for details.
EU11 Saturday, July 21, 2018 between 22:00 - 02:00 UTC
See page 20 for details.
Files Sync will be retired on May 25, 2018
SOAP API Legacy Versions will retire in Winter '19