Talking about Winter ’19 release while we are still in Summer 18 seems too early, however, it can’t be too early to learn about the new and exciting features coming to Salesforce.
Sandbox Preview
Whenever we talk about a new release, we always need to know when our Sandboxes are going to be upgraded. I cannot stress enough the importance of the Upgrade Calendar.
You can refer to the Salesforce Blog post here.
The Preview Instance Migration Date is September 7th.
The Non-preview Instance Migration Date is October 12th 13th.
This diagram should give you an idea of when the various Sandboxes are being updated.

Important Dates: Production Instance Upgrades
Though this is more important than Sandbox upgrade date, chronologically this event happens later.
The production instance release dates are September 8th, October 6th and October 13th.
Regardless of your Sandbox migration, your production org will be upgraded to the Winter ’19 release on one of these dates.
Enjoy the Winter ’19 Release of Salesforce!
As always, Keep Calm and Salesforce on!