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Learn how to navigate the sandbox preview process to ensure that your sandboxes are ready for Summer ‘21. The Sandbox Preview window begins on May 7, 2021. You must have an active sandbox on a preview instance before May 6, 2021 (the day prior to Sandbox Preview) to take advantage of the Summer ’21 preview. In most cases, you don’t need to do anything because your sandbox is already on a preview instance.

Summer '21 Sandbox Preview instructions