If a Scrum Team uses Product Backlog refinement, when should it occur?

Product Backlog refinement is an ongoing activity that can occur at any time during a Sprint, as needed. The Product Owner and Developers collaborate on this task, ideally in advance of the upcoming Sprint to ensure clarity and readiness of the work ©. If refinement has not been done in preceding Sprints, it should be done in the current Sprint to maintain the flow of valuable work (E). This approach is consistent with the Scrum principle of continuous improvement and the iterative, incremental nature of Scrum12.