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There's a built-in Completed perspective in the Perspectives menu. Open it, close all groups except the one for completed today, and save as a new perspective with a suitable name. Bring up the Perspectives window and make sure the Restore Expansion box is checked for that perspective. You'll still see the groups for completed yesterday, last week, etc. but they will be collapsed. You can add the new perspective as a toolbar button for one click access if you customize your toolbar.