We're kicking off Summer '21 in earnest today.  Earlier I posted about the new sandbox preview instructions on Salesforce Help - http://sfdc.co/Summer21-SandboxPreviewInstructions

You may be wondering - what are the other key dates I need to know about? Pre-release orgs, release notes, etc .. We've got a handy infographic for that.  We also have a Google calendar that you can view in any browser (https://sfdc.co/Summer21calendar) and/or import into your own calendar of choice (https://sfdc.co/Summer21-calendar)