Winter ‘18 began to make appearances on existing pre-release orgs on the August 9, with pre-release sign-up available on the 17th.
There is an open issue with Winter ‘18 preview releases affecting iOS users and MobileCaddy. The issue also affects users viewing Visualforce pages on iOS, using the Safari browser, and potentially other custom-built mobile applications. This issue was previously seen in the Summer ‘17 release but was patched by Salesforce and fixed prior to GA. The same issue has appeared in Winter ‘18. The issue has also been reported by other users and we have been in contact with Salesforce for further information.
Timings for Winter ‘18 can be seen in this graphic, released by Salesforce.
From time-to-time Salesforce undertakes a maintenance operation called an Instance Refresh. This is performed for various reasons, but primarily to maintain good performance. As part of this, your org may be moved to a new data centre. More information on this, and steps that might be needed, can be found in this Salesforce Help Document.
* Correct at time of writing.