Locate your Salesforce instance

1. Click on the setup cog (top right)
2. Type "Company Information" into the search
3. Select menu item "Company Information"
4. Copy the "Instance Name"

Navigate to https://status.salesforce.com/current/maintenances

Enter your instance name (1)  and press on the Search icon (2)

Click into result for your instance

Click on the Maintenance Tab (1)

The "Major Release" (2) are the updates that Salesforce performs 3 times a year.  This has the planned date/time for your instance.